Add coloured paper to the printer/ photocopier paper stack.
Randomly add coloured paper to the stack of papers in the copier drawers. Ensure that there are at least several inches of paper between coloured sheets. Individuals will have their print jobs thrown off by the coloured sheet, however, when the paper returns to white, they will assume it was an anomaly. People are lazy by nature, and will not check the paper if they do not have to. The lazy nature of others is the key in allowing the other sheets to do their job.
Some may not even notice the coloured sheets amongst their prints if they are the type of person who walks away from print jobs, or if the printer is located in an area they can not see printing.
In the best case scenario, this prank can be repeated over and over.