Having played a few TAKEBACK games, the features works very neatly as a training aid.

One improvement I would appreciate, though, is for a record to be (automatically) kept of UNDONE moves. Ideally, they would be flagged up in the MOVES history, itself e.g.

This would obviously mess up the game history, make it impossible to export, etc. (unless there is a way of adding comments?). So the obvious alternative would be to put a comment in the NOTES tab: 24 Rg3 TAKEN BACK.

Any thoughts?

You could still export it with the original move(s) being stored as variations in the pgn.

If the pgn file has the variations in it then it would be fine. I dislike it that if you want to jump back a bunch of moves it takes quite a bit of time.

simple method is to track the game offine in something ike chessbase and note the takebacks as variations. The complete pgn could then be posted to the student as a complete lesson 

this would save erik and his overworked gang from any extra work for now.

I wonder: how many TAKEBACK games were started last month?

I started two

But your opponent can take back moves he makes without you knowing.

but as takeback games are usually for teaching purposes then why would your opponent do that??