I encountered this situation:
Trying to turn the last cart from the pile reveals another five of spades; i can hear the sound of the card turning, and the back disappears from the pile, but on the right there is no change and i can not drag the card.
However, the “Complete” button becomes enabled and it began by placing the two of hearts on the empty space on the far right; it finished placing all the card. It is like the one of hearts _is_ there, but i can not see it, and i did not see it any moment during play.