This has been rejected multiple times since it's first suggestion in 2006. There is too much room for this to just be used to test the waters on a bad strategy and then take it back because it was an 'accident.' The solution is to take the proper amount of time making a move so that you do not make a mistake.
A few things about this:
- This would take more than just an hour of coding. There is virtually no new implementation on this site that can be accomplished in that short a time frame.
- Having an undo button changes the game. This is especially true in freestyle.
- Basically, mistakes happen in most games (Cards, chess, board games, sports). There is generally no undo button unless you are playing against a computer.