This one is absolutely mortifying... but I'm glad it didn't end up worse!!
When I was 13, my parents and younger sister and I took a trip to WDW and the day we were in MK, it started pouring, so we got some sweet yellow touristy plastic ponchos to wear until the rain let up. We decided to go on Splash Mountain and kept our ponchos on so we wouldn't have to carry them through the queue.
About 20 minutes after we got in line, I felt like I needed to use the restroom, but there was no way I could walk all the way out of the queue, find a restroom, and then essentially cut everyone in line to get back to my family, so I thought I could hold it. An hour later, I'm about to wet myself as we get to the front of the line. I'm frantically looking around for a bathroom (as we're boarding the ride?

13 year old me was a genius) and realize that I have two options: board the ride or sprint through the chicken exit and hope that I make it to the nearest bathroom. I take option one.
As a last-ditch effort, I try to make it through the ride, but it's not happening. So I pull my poncho around my bottom half and let nature take its course

the poncho keeps it from going on my mom, who's sharing a car with me and has no idea what's going on... then before the big drop, I hike my poncho up to my waist and pray that we get soaked. Thankfully, we did, our car got filled with water and I'm drenched from the waist down.
The rest of my family stayed pretty dry and they kept asking how I got so wet... I just said my poncho must've ridden up when I sat down