OOOohhh! Biscuit Chucker, I do that TOO!
On one occasion, I have even done the reverse and mistaken a man for a lady!
He (or She, as I thought at the time) looked alot like Dame Edna - but not in the frock, just a pair of SLACKS! During service I asked "Excuse me Maam, would you like a drink?" I offered several times, but received an icy stare (arms folded) and NO answer. I thought to myself "Hmmm, why is this lady blatantly ignoring me like this? Talk about RUDE!" So I went and took a good look at the manifest and discovered that Edna, was infact, a "Sir" not a "Maam"!
It was all fixable!