As of December 5th 2008, this bug is now fixed. See: http://store.steampowered.com/news/2085/ You can still kill her from behind, but it's little bit harder now as she don't get stuck anymore, and will incapacitate you if you screw up.
As you can see, the witch get's stuck if you literally take her from behind...
Note this was recorded on expert difficulty (z_difficulty impossible).
Also note that this is obviously way harder to pull off if you can't reach the back of the witch, such as if she's sitting against a wall. Also seems like she can get unstuck if another survivor "helps" her with it.
I used "sv_cheats 1" and "z_spawn witch" to spawn the witch. No other cheats used in this video, and this can be easily reproduced in a real game. I also used kick so the stupid bots wouldn't ruin this video.