Noticing an awkward silence -- she was in a room full of LGBT and allies who didn't really give a shit -- I chimed in with "hah, sorry, I thought you two were a couple."
Suddenly, I'm hit with a self-important lecture on how there's more than three degrees of sexuality, how it's okay to identify as lesbian and date men, how I was being offensive, and how, if I had a problem with her 'nonstandard sexuality', I needed to just "deal with it."
...Naturally, as a gay male who was fairly active in the LGBT rights movement, I was perfectly aware of all of this. I was just trying to break an awkward silence to save her a bit of embarrassment, as I had (reasonably) assumed that she was distancing herself from her partner. I apologized, explaining my position, but with no luck --the two didn't talk to me for a few weeks afterwards.