This condition arises predominantly in situations where the catalyst is vile, repulsive or undesirable and against all better judgement, you get an erection.
A nasty trailer trash dance-queen drunk on lady's night well-drinks uses a fist of sausage fingers to grope your junk on the dance floor.
When you realize the sexy asian girl you are fapping to is actually a boy
when your cousin is forced to sit on your lap in the back seat of your dads car while driving on a bumpy road.
When you accidently brush up against a 90 year old woman in a swimming pool.