Thankfully these didn't happen on the same day, but I had spent weeks working on a hair prop for my costume. I spent the entire night before the con finishing it. My friend literally walked in to wake me up the minute I finished. We decide to check out vendor's hall first in the morning and I didn't even take more than 5 steps before some asshole checked my shoulder and knocked it to the ground, breaking it irrevocably for the rest of the day. The prop wasn't obtrusive in anyway, it was even smaller than the width of my body. He didn't even pause to say sorry, so I can only hope everything he wanted that weekend was sold out.
Another time, my bestfriend was just starting to get into cosplay and sewing. I had offered to help her a few times, but had a costume of my own to finish (another pulled all-nighter), so I couldn't help her at the end. On the day of the con, her stitches came undone and her dress was literally falling of her. We cobbled together some safety pins, bobby pins, and hair ties to keep it on under a sweater, but eventually she gave up and bought a cute outfit from the con instead. It ended up working out better than the costume which was a saving grace, but it sure learned me a good one to test wear my costumes before the con.