Let me begin by saying this is the straight-up weirdest bug I've ever seen in the game. I'm not sure how to best describe it. Basically, just run the following command on a Mac and play around with hitting cmd-a, cmd-c, ctrl-a, and ctrl-c in various orders before you enter it until you get it to spawn an invisible mob (detectable by "I" in the debug screen). Getting it to happen could take anywhere from seconds to minutes, depending on how lucky/unlucky you are. Then try /killing it, and you'll see "I" decrease even though there's no death message.

/summon Skeleton ~ ~ ~ {Equipment:[{id:bow}, {}, {}, {}, {}]}

The Equiment tag might not be necessary, but it's happened more often with it than without it. Also, for the record, I've tried reloading the world, restarting Minecraft, and deleting any invisible characters in the chat/command block line which I thought might be the problem, all to no avail.