Moisture... makes the paper fiber swell up, expand and if the holder is too tight it may make the face
of the card come up and stick to the case. Then, when you open the case you pull the card apart.
If you use screw cases be sure to use one a little bigger.
Most autograph cards are 15 points thick, the DE1 to DE6 and the Quad auto/costumes are thicker.
I use Pro-mold brand size 5II for autograph cards, good for up to 20 points. I also then put the whole thing in a team bag.
A point is equal to .001 inches, a standard measurement unit for paper thickness is "points".
Points is usually abbreviated "pt". No, it is not the same as in 'lets have a pint".

Context is everything.
