Simple little game, but works very well for what it is! (Would say, even better than quite some around here for its functionality)
The undo feature is a bit odd to have in a game meant to have chance elements, but it is interesting, and remarkable that it works, as far as I could see, dealing correctly even with more complicated cases. Using it turns the game into something different, of course easier, but only with good memory remembering the cards.
Abusing it I found a case which isn't really handled, resulting in a deadlocked game (always a bit curious what would happen in various spider solitaire implementations in this case), see as in the image. What happened here is that by assembling stacks, the board ended up having only nine cards left at this stage with a pile empty. Quite rare case, but possible :)