I've seen it a few times. Here's another one on Mission. When I rendered the clip it added the glass back into the window so it's hard to see.
It's not exactly a spawn killing but the guy was basically watching for heads to pop up over the wall and I believe he got 3 in a row.
http://www.youtube.com/watch?v=mP0jvuKydx4
It seems to happen when the floor directly underneath a spawn point is out of the enemy's view.
Almost as though the system is checking line of sight to the spawn but not to full player height, if that makes any kind of sense at all (I'm slightly drunken).
Just my observations. Needs fixing asap.