There has been some discussion in the BrainKing DB about the rules of Backgammon. There may come a time when someone who is not aware of the correct rules wishes to, in fairness to and in agreement with their opponent, retract an incorrect move and play something differemt. The same may occur in other games, although I haven't seen any such discussion.

Or another scenario is when friends (chess purists block your ears or close your eyes) play they may, by mutual consent, agree to one player playing differently.

I think you get my drift ... an undo move would be good.