I believe you can set up "takeback" as an option for unrated games.
A great Idea? - restart a game from JUST BEFORE a blunder

When you create a new game there is an option for "Rated", "Unrated", and "Takeback". A takeback game will not result in a rating change.

There is currently no mechanism to start a game from an arbitrary point - though it has been discussed ...
- does-anybody-think-this-would-be-fun
- new-games-with-predetermined-set-up
- handicapping-systems---levelling-the-playing-field
Many times we are part of games that end on a significant blunders by either ourselves or our opponent. When I am part of these games, it is frustrating not always because I lost, because of the blunder, but because what may have been a very good game was just ruined. I feel as bad for my opponent even if he/she won the game because neither of us get the full benefit from being a part of a great game. Here is what I would like to see:
1. I would like to be able to pick a game up at any particular point (in this case, right before the blunder) and continue it from there. This certainly should be done in an "unrated" or "for practice" scenario, and the great game could continue to see where it could have gone. I would like this very much.
Lee2000