It may sometimes be a scramble to find a socket to juice up your dying gadgetry, but the "first come, first served" rule stands fast in this area. You must never unplug another device that's on charge without first checking that it's okay to do so.
Always, always, always ask —and if you can't find device's owner, then walk away and find another outlet. For all you know, the gadget's owner might be about to head into a vitally important meeting or call and need all the juice they can get.