I was trying to figure out if anything in-game could have caused this, but I'm pretty sure it's a bug.
I was running around the "forest" areas outside of the Imp village in Act 2. I suddenly realized that I hadn't been attacked by anything in a while. The baddies were still there (dang tibars, ARGH), but every negative creature would run toward my characters, then stop a few "feet" away and act as if they'd just run into an invisible wall: legs moving, but no motion. I had my DK walk up and just smack 'em with his saber, and they never defended themselves.
This lasted through about a dozen saves and two game restarts. (Unfortunately, the monsters went back to normal when I loaded a savegame where I had been standing next to three previously harmless fire elementals. This time, they were active. Oops.) Since it's not doing it anymore, it's not vital, but I was wondering if anyone else had experienced the same thing?
(Oh, and yes, I am running 1.44.)