diff --git a/js/pacosako.js b/js/pacosako.js
index ff32862..474929c 100644
--- a/js/pacosako.js
+++ b/js/pacosako.js
@@ -31,26 +31,38 @@ const INITIAL_EDGE_ROW = ROOK + KNIGHT + BISHOP + QUEEN + KING + BISHOP + KNIGHT
 const ROWS = '12345678';
 const COLUMNS = 'abcdefgh';
 
+/*
+ * Rule Version
+ *
+ * Increment this whenever a change in the rules would make some previously
+ * illegal moves illegal. The implementation in the Game class needs to check
+ * this._version and apply the appropriate rules based on which rules were in
+ * effect when the game was started.
+ *
+ * Version 1: Initial version. (Default if version field is missing.)
+ */
+const CURRENT_VERSION = 1;
+
 function otherSide(side) {
    if (side === LIGHT) {
       return DARK;
    } else if (side === DARK) {
       return LIGHT;
    } else {
-      throw { message: 'invalid side', side: side };
+      throw new Error(`invalid side: ${side}`);
    }
 }
 
 function squareIndex(square) {
    if (typeof square !== 'string' || square.length !== 2) {
-      throw { message: 'invalid square', square: square };
+      throw new Error(`invalid square: ${square}`);
    }
 
    const column = COLUMNS.indexOf(square[0].toLowerCase());
    const row = ROWS.indexOf(square[1]);
 
    if (column < 0 || row < 0) {
-      throw { message: 'invalid square', square: square };
+      throw new Error(`invalid square: ${square}`);
    }
 
    return (row * 8) + column;
@@ -72,7 +84,7 @@ function encodePiece(side, type) {
    let result = ALL_PIECES.indexOf(type);
 
    if (result < 0) {
-      throw { message: 'invalid piece', piece: type };
+      throw new Error(`invalid piece: ${type}`);
    }
 
    if (side === LIGHT) {
@@ -80,7 +92,7 @@ function encodePiece(side, type) {
    } else if (side === DARK) {
       return result;
    } else {
-      throw { message: 'invalid side', side: side };
+      throw new Error(`invalid side: ${side}`);
    }
 }
 
@@ -92,7 +104,7 @@ function decodePiece(side, value) {
    } else if (side === DARK) {
       sideShift = 0;
    } else {
-      throw { message: 'invalid side', side: side };
+      throw new Error(`invalid side: ${side}`);
    }
 
    let pieceValue = (value >>> sideShift) & 0b1111;
@@ -100,7 +112,7 @@ function decodePiece(side, value) {
    if (pieceValue < ALL_PIECES.length) {
       return ALL_PIECES[pieceValue];
    } else {
-      throw { message: 'invalid encoded piece', value: pieceValue };
+      throw new Error(`invalid encoded piece: ${pieceValue}`);
    }
 }
 
@@ -108,7 +120,7 @@ class Board {
    constructor(original) {
       if (original !== null && original !== undefined) {
          if (!(original instanceof this.constructor)) {
-            throw { message: 'can only clone from another Board instance' };
+            throw new TypeError(`can only clone from another Board instance`);
          }
 
          this._board = new Uint8Array(original._board);
@@ -175,7 +187,7 @@ class Board {
 
       if (type !== EMPTY) {
          if (this._phantom) {
-            throw { message: 'phantom square is already occupied' };
+            throw new Error(`phantom square is already occupied`);
          }
 
          this.putPiece(side, square, EMPTY);
@@ -190,17 +202,17 @@ class Board {
 
    move(from, to) {
       if (this._phantom && from !== PHANTOM) {
-         throw { message: 'must complete prior move before moving other pieces' };
+         throw new Error(`must complete prior move before moving other pieces`);
       }
 
       let lightPiece = this.getPiece(LIGHT, from);
       let darkPiece = this.getPiece(DARK, from);
 
       if (lightPiece === EMPTY && darkPiece === EMPTY) {
-         throw { message: 'cannot move from empty square' };
+         throw new Error(`cannot move from empty square: ${from} → ${to}`);
       } else if (lightPiece !== EMPTY && darkPiece !== EMPTY) {
          if (!this.isEmpty(to)) {
-            throw { message: 'cannot capture with joined pieces' };
+            throw new Error(`cannot capture with joined pieces`);
          }
 
          const fromIndex = squareIndex(from);
@@ -214,7 +226,7 @@ class Board {
          const displaced = this.getPiece(moving, to);
 
          if (displaced !== EMPTY && this.getPiece(other, to) === EMPTY) {
-            throw { message: 'cannot join with piece of same color' };
+            throw new Error(`cannot join with piece of same color`);
          }
 
          if (from === PHANTOM) {
@@ -462,9 +474,38 @@ function diffHistory(game1, game2) {
 
 class Game {
    constructor(original) {
+      if (typeof original === 'string') {
+         const moves = JSON.parse(original);
+         original = new Game();
+
+         if ('version' in moves === false) {
+            original._version = 1;
+         } else if (!Number.isInteger(moves.version) ||
+               moves.version < 1 || moves.version > CURRENT_VERSION) {
+            throw new Error('invalid version');
+         } else {
+            original._version = moves.version;
+         }
+
+         for (const move of moves.past) {
+            original.replayMove(move);
+         }
+
+         let n = 0;
+
+         for (const move of moves.future.slice().reverse()) {
+            original.replayMove(move);
+            n += 1;
+         }
+
+         for (let i = 0; i < n; ++i) {
+            original.undo();
+         }
+      }
+
       if (original !== undefined) {
          if (!(original instanceof this.constructor)) {
-            throw { message: 'can only clone from another Game instance' };
+            throw new TypeError(`can only clone from another Game instance`);
          }
 
          Object.assign(this, original, {
@@ -488,6 +529,7 @@ class Game {
          this._undo = null;
          this._checkCache = {};
          this._ignoreCheck = false;
+         this._version = CURRENT_VERSION;
 
          /* set to false when the king or rook moves */
          this._castling = {
@@ -525,6 +567,18 @@ class Game {
       this._ignoreCheck = !!value;
    }
 
+   get version() {
+      return this._version;
+   }
+
+   toJSON() {
+      return JSON.stringify({
+         past: this._moves,
+         future: this._redo,
+         version: this._version,
+      });
+   }
+
    canCapture(side, from) {
       if (from === PHANTOM) {
          return true;
@@ -732,7 +786,7 @@ class Game {
       const kings = [...this._board.findPieces(side, KING)];
 
       if (kings.length !== 1) {
-         throw { message: 'there should be exactly one king per side' };
+         throw new Error(`there should be exactly one king per side`);
       }
 
       const king = kings[0];
@@ -894,7 +948,7 @@ class Game {
 
    move(from, to, meta) {
       if (this._status !== PLAYING) {
-         throw { message: "can't move, game is already over" };
+         throw new Error(`can't move, game is already over`);
       }
 
       const side = this._player;
@@ -906,13 +960,13 @@ class Game {
       const alongside = board.getPiece(other, from);
 
       if (from === PHANTOM && !board.phantom) {
-         throw { message: "attempted to continue a completed move" };
+         throw new Error(`attempted to continue a completed move`);
       }
 
       const fromSquare = (from === PHANTOM) ? board.phantom.from : from;
 
       if (!this.isLegalMove(side, from, to, alongside === EMPTY)) {
-         throw { message: "illegal move", side: side, from: fromSquare, to: to };
+         throw new Error(`illegal move by ${side} side: ${fromSquare} → ${to}`);
       }
 
       const move = {
@@ -1053,7 +1107,7 @@ class Game {
 
    resign(meta) {
       if (this._status !== PLAYING) {
-         throw { message: "can't resign, game is already over" };
+         throw new Error(`can't resign, game is already over`);
       }
 
       const move = {
@@ -1101,8 +1155,8 @@ class Game {
    }
 
    replayMove(move) {
-      if (!move || move.side !== this._player) {
-         throw { message: "other player's move", move: move };
+      if (otherSide(move.side) === this._player) {
+         throw new Error(`other player's move`);
       }
 
       if (move.resign) {
@@ -1164,6 +1218,7 @@ class Game {
    renderHistory() {
       if (this._history === undefined) {
          const replay = new Game();
+         replay._version = this._version;
          for (const move of this._moves) {
             replay.replayMove(move);
          }
@@ -1190,6 +1245,9 @@ export default {
    /* Coordinates */
    ROWS, COLUMNS, PHANTOM,
 
+   /* Versioning */
+   CURRENT_VERSION,
+
    /* Miscellaneous */
    Util: { otherSide, offsetSquare, diffHistory },
 };
diff --git a/js/pacosako_ui.js b/js/pacosako_ui.js
index fe46546..6ab0f38 100644
--- a/js/pacosako_ui.js
+++ b/js/pacosako_ui.js
@@ -50,7 +50,7 @@ $(function (){
       } else if (type === PS.PAWN) {
          return 'p';
       } else {
-         throw { message: 'unknown piece type', type: type };
+         throw new Error(`unknown piece type: ${type}`);
       }
    }
 
@@ -60,7 +60,7 @@ $(function (){
       } else if (side === PS.DARK) {
          return 'd';
       } else {
-         throw { message: 'unknown side', side: side };
+         throw new Error(`unknown side: ${side}`);
       }
    }
 
@@ -576,9 +576,8 @@ $(function (){
    function putState() {
       const boardElem = $('#cb_board');
       const gameId = boardElem.data('gameId');
-      const moves = { past: currentGame.moves, future: currentGame.redoMoves };
       boardElem.data('last_state', currentGame.moves);
-      putMeta({ board: moves });
+      putMeta({ board: JSON.parse(currentGame.toJSON()) });
    }
 
    function putMeta(extra) {
@@ -649,40 +648,13 @@ $(function (){
 
       cancelGameCallback = IO.onGameUpdate(newId, function(data, gameId) {
          if (data.modified > $('#cb_board').data('modified')) {
-            const board = data.board || { past: [], future: [] };
-
             try {
-               const moves = { past: [...board.past], future: [...board.future] };
-               const oldState = { past: currentGame.moves, future: currentGame.redoMoves };
-
-               if (!deepEqual(moves, oldState)) {
-                  debug('got board', moves);
-
-                  const newGame = new PS.Game();
-
-                  try {
-                     for (const move of moves.past) {
-                        newGame.replayMove(move);
-                     }
-
-                     let n = 0;
-
-                     try {
-                        for (const move of moves.future.slice().reverse()) {
-                           newGame.replayMove(move);
-                           n += 1;
-                        }
-                     } catch (err) {
-                        debug('Error replaying board redo state', err);
-                     }
-
-                     for (let i = 0; i < n; ++i) {
-                        newGame.undo();
-                     }
-                  } catch (err) {
-                     debug('Error replaying board state', err);
-                  }
+               const newGame = new PS.Game(JSON.stringify(data.board));
+               const newState = JSON.parse(newGame.toJSON());
+               const oldState = JSON.parse(currentGame.toJSON());
 
+               if (!deepEqual(newState, oldState)) {
+                  debug('got board', newGame.moves);
                   setCurrentGame(newGame, newGame.moves.length > currentGame.moves.length);
                }
             } catch (err) {