Ok this has happened to me i'm not sure if it has happened to anyone else but the new spawn egg plugin has allowed players to capture aggressive and harmful mobs.
More to the the point i have come across someone who has used spawn eggs to kill other people in a non PVP town using spawn eggs to kill other players, i have not got any evidence as i was not paying attention, until the player spawned a creeper,and i died. I thought that there were just mobs in the building before i saw the egg, if it is possible i would advise that you should not be allowed to catch harmful mobs because now non PVP areas are not really safe.