CP/M on a Superbrain
I was very lucky to have the opportunity to develop some comms software on a CP/M 8 bit Superbrain to capture the output from a PDP 11 (running a traffic control system) and save the output to floppy for later analysis. As my knowledge of CP/M grew I discovered there were a few bytes of the memory map unused. So I pranked a colleague's machine by inserting some interrupt driven code that inverted the colours on his (monochrome) screen once every minute. His reaction? Just carry on as though nothing had happened!
I suspect that would be frowned upon nowadays...