It's more common than you'd think, especially for park riders. If you land on your butt hard enough you can trigger a spasm.
I've had the spasm happen twice and I had to clench insanely hard both times to avoid a very messy walk home. Never had anything come out, but pretty dam close.
This.....never actually crapped myself yet, but damn sure thought i did..so much so that i went to the bathroom to check, but was thankfully clean
it hurts like nothing other