He was basically flashing a numbered ticket he has that has "1" on it, it was a ticket at an event this past Saturday and he was stressing how he was the first person there(it being a big event, being first did matter), so I said "Ah du ma!!" (Vietnamese for mother f*cker) and out of reflex, he said "Ah du ma may!!" (Vietnamese for f*ck your mother) at me.
Then for that moment, we were both like "WHAT!?" and we just cracked up for several minutes.