(This one just seems appropriate for October 31st!)
When I was in high school, I was a member of the choir. In addition to the "serious" pieces we did throughout the year, every spring we did a "Top 40" production: a montage of songs that could have been heard on the radio at some point in the previous few decades. There was music, dancing (or choreography, at least) and a lot of fun.
During one particular year, I was given the solo for "The Monster Mash," and we went all-out. I was given a lab coat, some other members of the choir got monster costumes, and we even had a vampire to come out of a coffin. We must have spent months rehearsing the production, but because the sound equipment had to be rented, we only had the microphones and speakers we'd be using for the big performance available on the day itself: not long enough for a full run-through.
You can probably guess by now what happened. My "big number" came up, and I grabbed the microphone, and hammed it up for all I was worth. Unfortunately, no one had told me that the microphone, being a wireless, had to be turned on to be of any value whatsoever. This meant that although my parents and everybody else in the audience could see me relishing the role of mad scientist, no one could hear anything I said! And we only had the one performance, so there really wasn't any opportunity to "fix" the mistake at a future time.
I've always wished I could "go back" and redo that performance. It really was a lot of fun putting it together in the first place. I certainly know a lot more about sound equipment now than I did back then. Still, sometimes you only get one "make it or break it" chance. That one just wasn't my day.