OK, I've got one I've never seen. I'll try to reproduce it --
I was playing Yukon in the latest Beta and had one particularly long stack. When I later dragged it to another stack, I was surprised to see 3 kings in it when I thought there had been only 1 (the rest of the stack was off the screen). I was then REALLY surprised to see two of them were the king of clubs. I currently have 6 kings showing as well as numerous other duplicates (see below). I was unable to finish the game because I got to the point where it wanted to play the 7 of clubs to the stack and I had already played the first 7 of clubs. Everything played normally except I had duplicate cards. Even the counter at the bottom was normal except it said I had 57 cards remaining when I had already played some to the Aces.
Game #: 957938
Currently showing: 2 of the following: KC, KS, QD, QH, JC, 10D, 9C, 8H, 8S, 7C for 62 total cards