I had my hu-cards in one of these, looks like it may have done something similar to some of mine.
it looks like the plastic softens the clear coat on the cards, i wiped them down with a damp tissue and left them with tissue in between each. i think it may be hardening a little, not sure yet as it has only been a week since i took them out.
this thread should be stickied, there must be lots of people storing there hu cards this way