Jumping onto a bicylce before you realize it doesn't have a seat.
What's grosser than that?
Getting off.
Here is our winner
My first bike was handed down from my 11 years older girl cousin and the seat was missing. The fact that it was a girls bike was OK because they didn't make kids bikes when I was learning to ride in 1947 and I couldn't have reached the pedals from a seat anyhow. I never asked why they didn't get a seat for it as I grew, that never occurred to me. There were many things I never asked about that would have bothered me for my own kids. The wooden handle on my hand pump car broke so I just used it for years by grabbing the vertical metal post. It would have been very easy to cut a broomstick to replace the handle but when I was 5 I didn't know that. The school I was going into for the 5th grade was far enough from home so I needed a bike to get there so the summer before I got a new boys bike from the Western Auto. They did have a school bus but my house was a couple hundred feet short of a mile from the school so I wasn't allowed to ride it.
Now tell us that gross incident....