I actually have a couple, but I'll start with one of the worst.
Me and 15 other friends wanted to do an entire danger girl costume stage show at comic-con masquerade. We spent almost 2 months building costumes (almost everything was from scratch) making an audio track, complete with professional mixing and sound effects, and choreography for the whole thing.
2/3rds the costumes went ahead with most of the women sewing up the last stuff in their hotel rooms the night before while me and my friend where literally taking shifts sleeping while the other worked to sand and paint the last pieces. I made a communications backpack from junk in 3 hours when we realized it was forgotten in the chaos. I slept during the drive as my friend got us there the morning of the masquerade and we barely got checked in as finishing touches were made to everything.
NOW COMES THE BAD PART! I don't know if they do this for every year, But all the contestants were shuffled into a TINY room with no heat almost an HOUR before the masquerade started. Then we all had to wait as they called everyone up on stage (we were near the end of the whole thing) and then we had to WAIT ALMOST ANOTHER HOUR FOR THE JUDGING AND RESULTS!!! The final tally that I remeber was almost 4 hours of waiting to - LOSE!!! No honarable mention, no fan favorite, no nothing. My friend who arranged the whole thing was so pissed he almost punched a few people who told him he had to wait till he could leave the "green room"
Now for my personal part of it, I was dressed head to toe in a heavy foam and leather costume that had no ventilation or fans, I was sweating almost as soon as I had it on and I stayed in most of it for the unbearable 3 hours of waiting, then we get up on stage and I had a mask that I couldn't really see out of and I had no time to seal the LED leads next to my face, so I was getting little electric shocks from the sweat touching the HOT wires and my FACE! Then I hadn't the time to figure out how to hold the helmet to my head, so I had a hat on that barely held it on. I then made a fatal mistake in timing on the choreography and hit the hat right off my head! I held the face on and got thru the rest of the stage set, but I personally think I lost us the show.
Then during the wait for the judging and such I realized I wasn't sweating anymore, which to anyone who had worn a costume for any length of time knows, I was close to dehydration and shock. As soon as we knew we hadn't won we raced to get me out of the outfit, but it was too late, I passed out while walking and they carried me to the hallway and then got me some more water to wake me. I barely got outside and called my friend (who I was staying with) to come and get me, I then puked on the sidewalk and almost passed out again. When we got back to my fiends house he asked if I wanted to go to the ER, I said no and then passed out on his living room floor.
I barely remember the next day other then my friends telling me it wasn't a total loss, The creator of danger girl gave us a personal high five (via e-mail) and we were hunted by fans who wanted to know who we were. There are still some days I mention this and some people remember seeing us and how cool the whole thing was to them.
Half the people from the group never cosplayed again.