Wrong Instinct
To help out in the fight against the Drow and Grimlocks, the Mage, a specialist Conjurer, summons up yet more creatures to aid us in battle. This time we get some Celestial Riding Dogs. Once they are called they get to work attacking the Drow ahead of us. The first dog attacks, hits, and deals some solid damage to the Drow. The DM crosses off the hit points and tells the Mage that the Drow is dead. With a little cheer, the Mage picks up the miniature from the mat to remove it from the game, except he picks up the wrong miniature: he takes one of his summoned creatures from the table instead.
I guess he's just too used to dealing damage to the creatures he's summoned or members of the party instead of the enemy, and it was an instinctive reaction.