This has happened a few times, where I will go to drag a card from the Tableau to the Foundation piles, and it gets stuck, somewhat offset to the top right from the cards below it, but then you can't interact with it. It prevents game completion. I will take a screenshot next time it happens and post it, but I figure, if that rings a bell, at least you have the bug report to start with, lol.
Also nice game, :D