This happens to us all the freaking time. We’ll often get half way through a map and someone will point out “oh, don’t they have flying? Oops…”
The other common one is ranged attacks. Turns out one of the monsters Oozes. (I hate oozes with a fiery passion only known by those who’ve had to do all the admin involved when they spawn three times in a row.) has a ranged attack. We managed to get through an entire mission and loose by a minus 1 card before we realized they had a ranged attack.