I've noticed, and probably alot of other people have, that zombies tend to close the last meter in between them and a potential target by 'teleporting' over the distance .. which sometimes leads to them be able to warp through doors and walls and sometimes hop obstacles.

Also, they tend to teleport to you when you're just peaking over a ladder, making zombification iminent and then you also get stuck, because zombies can't climb ladders anyways and you have about 10 of them stuck onto you.

I've noticed the teleporting thing on some servers, always thought it was just lag

Yeah this happened to me too. On the map "bb_dotd_mall_final" when you try to climb the ladder down to the sewers. The zombies will teleport and get stuck on you. And they turn me into a zombie and I fall in the sewers and can't get out. Also on the same map, sometimes when i got to "Andy's Gunshop" The door to inside will close but some zombies will teleport through the door!

Another thing, on stronghold, if the zombies get close to the moat, thery teleport over the edge and partway down the slope of the moat. Regardless if you are close or not....

It seems that anyplace where zombies CANT cross or trasverse over (such as ladders, moats, sandbag fortifications, and the like) they seem to close the last distance in between them and the target by teleporting. Also, they tend to get stuck on you when that happens .. I've seen it happen to those poor marines all the time.

Edit -> also, in bb_dotd_mall, zombies tend to walk halfway through the glass doors, making them shootable from the otherside.

Zombies can teleport through the fences surrounding the base on slaywatch. Its common with the HL engine though. THat's how some players end up in the "void" zone on a map.