In a line at the store, waiting for the bus, or even while sitting at the theater, there will be people.
With people, things happen. =\
Some of these things might have happened to you...
Assuming that the person doesn't have a bowel problem, they are somehow completely unaware* that they sharted... D:
*If you point it out they will claim it is someone else that's sh*tting down their leg. ◎-◎