Accidental, unintentional contact between your naked arse-cheeks and another individual's naked arse-cheeks, resulting in a panicked moment of pause, sudden development of perfect posture on bolting upright, butt-hole puckering and the grunted "sorry" cough being elicited. Commonplace at such establishments as gyms or work locker rooms.
Two individuals standing side by side at neighbouring lockers.
Individual One: *turns to the right, bends over*
Individual Two: *turns to the left, bends over*
Arse-cheeks touch. Moonlanding has occured.