The most frustrating, sincerely upsetting experience I ever had was when I was about 13 or 14, and encountered the following code: (roughly)

   10 for x = 53248 to 57343
   20 poke x, peek (x)
   30 next x

When I saw this code - it caused physical angst on my part, as it was obvious that this code did nothing. I knew nothing about memory mapped ROM/RAM banks at the time, but I quickly learned....

I've never had as much fun programming as I did with that Commodore 64 and the Programmers reference manual (and all the Compute! secrets/tips books) - it was the youtube minecraft videos of our time.