Every time I start out at a new place, I get mistaken for a secretary (or an undergrad, but I'm starting to get old...).
Today, some guy barges into my office (even though I had 1 cm of door ajar), and demanded to know where Professor So and So was. I calmly asked him to go ask the undergraduates that man the front desks, and the man still wouldn't leave! I had to escort him out of my office and point him to the front desk.
Do I look like a secretary? Oh wait... I forgot. I'm a woman. Therefore, I couldn't possible be a gen-u-wine professor. I must be a card-carrying-index-filing-coffee-making-bitch of a secretary who happens to write a lot of research proposals dangit.