Being on the XXXS side of things, I've had this happen to me more than once. My boss at my old job decided it would be a great idea to make all the doctors wear uniforms (scrubs). All the other doctors were large men. My scrubs didn't even come close to fitting me, and had an elastic waistband. Twice while I was scrubbed into surgery, my pants fell down. You can't do anything...you can't touch anything that's not sterile, you have to keep going. At least I had a gown on, but I was NOT amused.
Needless to say, I don't work there anymore.
The problem isn't that I don't have an ass. It's that I'm hipless, thin and narrow. My shoulders are the widest thing on me. Elastic waist pants no workey.