involuntary moodslaughter
One of the many degrees of mood murder. If one is not intentionally trying to ruin someone's mood but manages to do so anyway with a stupid remark or a lame joke he is said to have committed involuntary moodslaughter.
Guy #1: Dude that dig about vegetarians was mood murder. you know my girlfriend is vegan.....
Guy #2: I had no idea she was sitting there. It was involuntary moodslaughter at best.......
Guy #2: I had no idea she was sitting there. It was involuntary moodslaughter at best.......