Heh, I remember the old 10 print "fuck off" 20 randomize usr 1331 trick. Many happy lunch hours watching the staff at Menzies fiddling round the back of the display Speccys trying to yank the power cord out to shut the thing up.
Anyone try running a 10p across the Amstrad 6128 edge connector?, amazingly, it didnt knacker it like it would have done on any other machine of the era, but produced all sorts of colourful displays and random sounds.
The menzies near me was great, the display machines were far enough away from the checkout that you could get away with all sorts. I used to ask to try c64 games, which they would stick in the tape deck and leave you to it, then I would power off the machine, insert a freeze machine cartridge and copy the game onto a blank tape. Happy days indeed, although one guy got caught doing it and got quite a severe fine after being escorted away by plod.