Depends on the superhero.
Superman: it would burn a large hole in the ground, because his feces is made out of molten uranium.
The Flash: who cares? It would be over quick.
The Human Torch: He'd just incinerate it and go about his *other* business.
Robin (not a superhero...): Well, it's not like he wears pants, so I guess he'd just drop it and keep on runnin'.
If you were to cosplay as a large box, what large box would you be?