I have a player in a Pathfinder campaign I am DMing that caused the game to come to a complete halt for nearly 20 minutes before I could get him to accept the rules of the game.
His character is a Summoner with an Eidolon who lagged behind when the fighting started on this particular encounter. He naturally wanted it to catch up in one move, but his positioning wouldn't allow for a Charge or Run action. Thus he took a Double Move (replaced his Standard Action with a second Move Action) and made it all the way up to the enemy, except he had 10' of movement still available to his Eidolon. This is where he tried to say that the Eidolon attempt to tackle the enemy in front of him (I am assuming a Bull Rush or Grapple action). And that is when we spent around 20 minutes going back and forth on why he can't; from how a Double Move is in essence a full-round action to how still having 10' left does not mean he can have a free third action. He eventually conceded to this but got noticeably quieter throughout the rest of the session.
Going over what happened a couple days later, I recognize that I may have gotten a bit snappy about half-way into the argument after getting somewhat tired of paraphrasing myself; but I don't want to be the cause of someone not having fun in my game. He has done similar things like this before in previous campaigns and I am predicting something like this will happen again.
So the question comes down to: If this situation is to happen again, how should I handle it?