Out of the mouth

Okay so there are a guy ad a girl. The guy says to the girl "thanks for last night". The setting: the kitchen of where I work. The players: me and someone I work for. He was saying thank you for helping with the event last night at work (the one where I was supposed to work 5-9 but ended up there til 10), but it was pointed out to me that someone not knowing the situation could derive alternate meaning from those words. Appearently someone had given him a weird look. I find it amazing that the simplest of things can be taken wrong.