[bcoates]

I always hear people worrying about this, but it doesn't sound likely at all. Has it ever happened?

[maxerickson]

Not quite the same thing:

http://www.policemag.com/list/tag/friendly-fire.aspx

(Do note that many of the incidents there involve an officer intentionally firing on other officers. But there are accidents there.)

[nwh]

I have no idea. I live in a country without the general public having access to handguns.