I have seen many an offsides on a corner. Usually, it is the kicker. He/She kicks it infront of the goal and all the defenders step towards it, then the kicker comes and plays off a pass or deflection and is offsides.
Also, I do not see this called often, but technically if a player is considered a threat the goalie must be mindful of, that player could be called offside even if he/she does not play the ball. I can imagine if a player were marking the goalie, and the post defenders were a step or two off the posts, then that player may be offsides.
Just my two cents, since I did not see the game at all.
