Or the paper ticket is even slightly bent. Or bent in half. Or mildly wet. Or a corner is folded. Or sometimes it looks fine but just won't work. Or it works at one station but not another.
I say all that because each situation happened to me on my visit to Barcelona this summer :)
I also had a problem where I had to basically rebuy a new 10-trip ticket twice within a day of each other, because the first got folded in a bag, and the other I thought I misplaced (only to find it at the absolute bottom of a heavily packed shopping bag). Oh and the one day I tried to get my folded ticket replaced, the only station I passed through was closed early! How convenient :)
Basically, those paper tickets WERE a pain. But I don't like the new photo ID cards they're introducing either. Good luck I suppose...