Nothing will compare to being told that I had ruined a child's Christmas when one of said child's parents came into the shop where I was working shortly before closing on Christmas Eve only to discover that we had (shock! horror!) sold out of the product that was at the top of the kid's Gimme Gimme Gimme list. Yes, that's right, all my fault.
Oh, something similar happened (again on Christmas Eve, shortly before closing) when a customer discovered that the supermarket I was working in had sold out of turkeys, mince pies and some other Christmassy foodstuffs.
And going back to the first job, I was also personally responsible for ruining some little brat's festive season when it transpired that there was a fault with the product and it needed replacing - only we didn't have any left in stock on the first day of opening after Christmas. Daddy Dearest had already threatened me with physical violence so I called a manager and refrained from pointing out that the kid really must have had a lousy Christmas with only the one present and him for a father...
Edit: Hmm, I seem to have missed the boat re "jolly japes" . Ho bloomin bah humbug