In the new sewer area, the zombies there like to fail to actually crawl all the way up, for the joining player. My host didn't seem to see the problem on his end. I was applying knockback (crossbow explosions) to the zombies as they were climbing, if it matters, though that didn't visually seem to affect them.
I even saw a zombie climb from the sewage straight into a wall one time. Not sure if related.