Accidents aren't usually good things. I mean, even if a couple gets unexpectedly pregnant they call it a "happy accident."
The word's usually reserved for unfortunate occurrences and things that shouldn't have happened. Like when you walk in on your parents getting it on, or when you back your car up after dropping your grandmother off, forgetting that she likes to walk behind the car and not in front of it. Sorry, Nana.
But as it happens, there was a recent happy accident of the non-pregnancy variety that occurred on the r/CasualUK subreddit. The fluke resulted in a Reddit treasure hunt that a ton of people are partaking in now.