If the target of detention sphere (one of the goblins) is not there by the time its ETB trigger resolves, it will fizzle and do nothing. One goblin will be in the hand safely, the other will be on the battlefield still.
November 28, 2014 5:40 a.m.
It depends on which Goblin you Snag. If you bounce the one being targeted by Detention Sphere's ability, the ability will be countered for lack of a legal target. If you bounce the other one, the one being targeted will still get exiled.