I don't remember the exact steps, will try it again tomorrow but I was frantically pressing left to move a card across the screen. It happened twice in a period of time and I believe two different coloured cards as well.
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I tried a couple more times, it only appeared once again and seemingly late game. It would happen as the card was falling, around the 2nd row from the top on the first column on the left. I believe in this case it was a light blue card and it just completely melded with the background colour. In the bug I experienced before, the card was still "there" and cards could stack on top of the invisible card. Perhaps it's to do with pal()? If that's the way you're producing all these colour cards.