Companions can't inflict damage in the ogre cave
Companions/pets have an exceptionally hard time killing ordinary ogres in their cave.
Problem
Companions attacking the ogres in the ogre cave deal damage at a much lower rate than usual, if at all.
How to reproduce it
Having a companion/pet powerful enough to dispatch ogres, receive the corresponding quest from Thrundarr. Then, go to the ogre cave and command your friend to kill one of the ogres. They will seem to always miss or fail to hurt them.
Notes
The Guidebook says the bug prevents them from inflicting "any" damage. During testing, my greater earth elemental pet that could two-hit emperor liches, was able to kill an ogre, but it took it around 15 attacks.
Other monsters (created with a wand, or wished-for) in the ogre cave seem to take damage normally.