A lot of the really young kids (4-5 years old) have trouble with when they're supposed to say "ma'am" or "sir". For example, we have a female instructor that starts off most of the little kids classes. When she's not there, about half of the kids will call me or the male Master "ma'am" because they're used to saying "yes, ma'am" at the start of class. When they mess up, we either laugh or just gently correct them.
A lot of the older kids mess up every once in a while. When they mess up, I give them a hard time.
Every once in a while I'll mess up, if I'm expecting a woman to say something, but a man does instead, I already have "yes, ma'am" primed in my brain and it just comes out.