A common fan service/comedy trope wherein a guy (or girl) accidentally does something lecherous, such as grabbing a breast or catching sight of someone in a state of undress.
Accidental Pervert is a common trope found in all types of entertainment media, used as a form of fan service and/or for comical effect. It typically involves a character, who may have the purest of intentions, accidentally doing something "perverted" or "lecherous".
Examples are usually found in anime and manga with fan service themes aimed at teens and adults, harem-themed series in-particular. Common situations include accidentally touching (or grabbing) someone in inappropriate places, happening upon someone (or multiple people) in a state of undress, or walking in on someone using the toilet or bathing.