Remember this well. Though my three line programs were a little more mischievous. When Boots the Chemist weirdly started selling computers I thought it was fun to go in on a Saturday afternoon and type:

   10 PRINT "Boots is rubbish, shop at Dixons.";
   20 GOTO 10

I thought I was a genius. Probably should have got a slimy career in adtech after that, but thankfully I didn't.

the WHSmiths shops had some of their computers sound amplified through the cassette players

a mischeivious program of

  10 pause (big number)
  20 beep
  30 goto 20

it would sit there for ages until a key was pressed, then beep very loud when a person - who's very first touch of a computers keyboard, would seemly set an alarm off.

the faces of the customers thinking they've broke the computer is still etched in my mind

    *FX 200,3
    
    10 *MOTOR 1
    20 *MOTOR 0
    30 GOTO 10

And then leave the shop quickly as the BBC Micro's internal relay quickly destroys itself.