Invisibility — don’t we women professionals *all* take it for granted that we will be assumed to be the +1, in a more-social networking situation? I have never had someone assume that I was the invitee, and my companion the +1, unless I was wearing a speaker badge. And sometimes, even then.
Another good one: when women travel (even on the SXSW nerd bird), men continue to discuss confidential information in your presence, never imagining that you know the people/companies being dished on.
Bonus points for mom: Get ignored even more completely when you have a small child in tow. (Many professional networking events are geared for families.) With a kid it’s inconceivable for most that you are anything but a wifely +1. Even other women will ignore moms with a child in orbit.