Funny, my first was similar, in Apple ][ BASIC. When I was in 7th grade, a friend (still to this day) and I had a typing class which used Apple ][s. One day we had a substitute teacher in who knew absolutely nothing about computers. I decided it would be funny to prank her so I wrote something like:
10 PRINT "GOVERNMENT ACCESS ONLY: THE FBI HAS BEEN CONTACTED"
20 FLASH
30 GOTO 10
I called her over with a worried face and said "I don't know what happened!" I could see her heart skip a beat as her eyes bugged out.We quickly started snickering and admitted it was just a joke. She laughed it off and was a good sport about it.