Worst to date was for my first con, when at about one in the morning the night before I was supposed to fly out, I'm trying to hem the skirt for my Road Kamelot cosplay. (Never hem your own skirts. Srsly.) Anyway, I get it all pinned, and then I go to cut down the fabric so the hem's less crazy-huge, and SLIP go the scissors, up high enough to basically make it the kind of error I can't fix. (I was Road in pants for that con. Much vexation.)
Minazuki's first tsuba broke not once but twice at AX last summer.
I somehow managed to make the collar on my sister's black shinigami kosode wa~y too short somehow. This was -- once again -- at the wee hours of the morning the night before going to con, only this time I was the driver, so I didn't fix it. It turned out to look okay because she's tiny, but I'm definitely fixing that for next time she wears it! (I also, same AX that destroyed Minazuki, managed to faceplant the Tenipuri plushie I was making her into a puddle of paint. There were jokes all throughout con about how I'd decapitated her doubles partner.)
I guess the only good thing I can say is that I've never really injured myself making cosplays, aside from the usual minor oven burns and pinpricks.