I found out this weekend that if you keep metro tickets in your wallet next to a pile of coins, they stop working when you insert them in the ticket barriers. This is a fun way to annoy hoards of impatient Parisian commuters as you try out a whole carnet and none of the tickets work.
I also found out that if you take the damaged tickets to the ticket office, they will not simply replace them but actually re-magnetise them for you (or at least, I think this is how it works) by inserting them in a machine one by one. This is a really fun way to annoy the grumpy woman behind the ticket desk. If you really, really want to piss her off, though, make sure you smile brightly and wish her a “très bonne journée” and comment on how “la vie est belle” as you depart.