Years ago I was teaching in Japan and I had a new business class to teach early one morning in Tokyo. New class, new room. All men, well boys really.
I had written some bits on the board and got everything ready. A couple of the students came in and sat down, sooo polite, quite basic English. Say hi, another couple of guys arrive, all looking super smart arrive and then one more.
He sits down and is the last one on my register so I get up from my chair to shut the door, however my foot ( and I remember I was wearing wedges) somehow gets stuck behind one of the chair legs and as I stand up I come crashing down full bodied onto the floor, hand in the air still ready to close the door 
I was mortified. Hadn't even introduced myself. And the students, love them, being typically Japanese, were mortified too and couldn't do enough to help me up.
I have also asked a colleague when she is due in front of other colleagues. I will never, ever ask anyone ever again if they are pregnant or when they are due.