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.

Step 2: Locate the Cables for the Coinbox

Locate the cables leading from the coin box.
The 4 conductor cable (black, red, white, green) is what we are interested in, while the 2 conductor cable is connected to a wallwart supplying power to the coin collection box.

Step 3: TEH H4X!

Now take your paperclip out of your inventory, apply your shape/craft skill to create a narrow "U" shape.
Insert one prong into the connector conduit containing the black colored wire, and then the other end into the red.

Step 4: Verify

Now that your paperclip is securly in place shorting the red and black wires out of the coinbox, check the copy machine's screen. It should now have different paper sizes and let you change the # of copies you are allowed to make.