Alternately, if no attempt at control is made (like the troll in the gatehouse scenario), perhaps the summoned creature seeks out the summoner for revenge or a ticket back home.
Parameters would need to be established to prevent an opposing player from simply picking up the story, (as the summoned-but-not-controlled creature is now an NPC), and playing it off as a free attack on the summoner. I certainly would. You summon an uncontrolled fiend, my turn comes around and I establish that the fiesty fiend, Rho Zyo Dun'l, is pissed at being whisked away from rummaging around in someone's larder, and looking to take it out on you.
Uncontrolled summons would have to be controlled by a neutral third-party GM, or separating the summon/control would never work in a game designed like this.
