I am currently running a teaching game with another player. They report that they were only able to Undo a single time, and that afterwards the Undo button was no longer visible. Though I swear I remember playing in matches where we performed Undo several times to back the game up to a specific Move. Can anybody confirm if there is a limit to how many times you can perform Undo in a match? The match is unranked, if that matters.
U P D A T E
You can chain undos until the start of the match if you both want. If one undo is accepted the button disappears for that player, that is true of course, you can’t really ask an opponent to undo his move 
Now the other player has to ask for an undo if they want to move back multiple moves.
Is it possible that this player has encountered a bug? They were able to undo once but are claiming that they no longer see the button. I’m not sure how to help them further, given I already know they can recognize the Undo button. Is this something a moderator could help us out with? If so, this is the game, and we’d like the game to be rolled back to Move 12. You can see us discussing the undo situation in the Chat log. Hazy confirms his desire in his last message in chat.
Yes, that is the expected behaviour.
Probably did not make myself clear, sorry.
He asks for an undo, then you ask for an undo. You have keep switching like this, because your opponent cannot ask to undo a move that you have made.
I cannot “edit” games in any way, sorry.
Alright I will let him know. Thank you very much for your help AdamR 
That’s kind of weird tbh, maybe Mulsiphix did not yet accept the undo, or the board state changed? At least I never heard of the undo button not being there :/.
Since it is unranked anyway an easiest and quickest fix would probably be to fork the board from the desired position. Just navigate to the variation you want > click fork > and issue the challenge to the original player.
By the way, if he ended that game, downloaded the sgf and used it to start a new match, would this one retain the comments in the chat? This would be a good variation of what you suggest.
hmmm I am not 100% sure, but I think forking the game wipes the comments unfortunately for their case.
Just hopped in the match and an Undo request was waiting for me. I’m guessing the button finally showed up for him. Hurrah! Thank you for the feedback guys. Turned out to be a false alarm. But hey, I learned something new. Moderators cannot affect the turn state of a game. So there is that 
Forking AFAIK also wipes Komi, not sure about captured stones.