What are some harmless ways to prank someone?

Put #define true (rand()%1000 != 0) in some obscure C or C++ header file on their system/project. This makes true evaluate to false one in a thousand times, and will make them spend hours trying to figure out why things like infinite loops, aren't quite infinite.

Other languages should also allow you to do things like this, if not messing with constants, messing with standard library functions.