This proof of concept instructable will go over how to exploit a popular pay-per-copy type machine with a paperclip.
Step 1: Find the copy machine and a paperclip
These machine's LCD will say "Set Key Counter"
I found this system in 2 seperate college campuses, and I usually keep a paperclip in my wallet, very usful little guys. Paperclips are so freaking useful, PROTIP: keep one with you.