Guru meditation error
I thought the Amiga Guru meditation error was nice, so I put that into the gift card balance check for a site I worked on. The card processor used JSON (if I recall correctly), and return reasonable text error messages. So I had it print a custom message for invalid card number (it would also lock the page out for 5 or 10 minutes if they put 2 or 3 bad numbers in), and custom message for one or two other "normal" conditions; otherwise it printed "Guru Meditation Error" and directly printed whatever error message they sent, and then would suggest trying back in a few minutes. Not really a funny story about this, just a shout out for the guru meditation error 8-)