Most of game corruption wasn't the result of faulty replay save / generation. Most of game corruption was the result of replays only recording user actions, and when the game engine was patched and rules were changed, re-playing the same user action could go wrong: imagine the cost of Probe getting increased from 50 minerals to 60. You load a replay that contains a "Build Probe" command when the user had 50 minerals. In the new game engine no probe will be built as the player does not have enough minerals. If this new probe was used to build other buildings, those also will not be built in the new game engine. Butterfly effect.