This happened when I was 9. My parents were getting us ready to leave, but I had incoming diarrhea. I lined up for the bathroom, 3 people ahead of me. I was desperately holding it in for the next 5 minutes. When it was my turn, I ran in, locked the doors and pulled my pants down...but I did not make it to the toilet.
There it was in the middle of the bathroom, a piping hot chocolate soft serve. I grabbed a bunch of tissue and scooped some of it into the toilet and left a few smears on the seat. 9 y/o me panicking while the next person is knocking on the door. Also at the point, my parents have been waiting for me for about 10 minutes already. I washed my hands, left the bathroom and pulled the door close as I exit so I won't get an immediate reaction. I then ran out of the McDonald's.
I am deeply sorry to the staff that had to clean my mess and the person who tried to go in after me.