One day while I was on maternity leave, I was having coffee with a friend. A woman, who looked very familiar, came up to me and our conversation went similar to this: "Hi, C! Great to see you, how are you? Is that your daughter? Wow, my son is 10 months now, I'll be going back to work in a couple months, etc. etc." Meanwhile, I'm sitting there, totally drawing a blank on who the hell she is. I want to be polite and introduce my friend to her, but I have no fricking clue how I know this girl. So I just carry on polite conversation, trying to phrase questions where she might give up a clue as to where I know here, but no dice. So after a few minutes of that above conversation she she says goodbye, leaves and I admit to my friend that I have no clue who she was.
Three days later, it finally hits me who this woman is. A coworker. From my department. Who I'd known for about four years.
I'll totally blame my sleep-deprived baby brain on the gaffe. I have no clue if she ever realized that I had no clue who she was - if she did, she never mentioned it when I returned back to work.