On my Spectrum I came up with: 10 GOTO 20 20 GOTO 10

It would start the Spectrum's "loading screen" border which would then scroll infinitely.

At some point I also opened it up to see what it looked like on the inside, but I managed to damage the keyboard in the process. Thereafter whenever I wanted to play a game (via joystick) I would have to use a piece of wire to manually select the rows/columns of the matching keys to enter the load command.