Recently I got a time to get back to ROT, so I finished ROT3 and started Anthology (Weapon of War) campaign.
I faced some odd things, and it looks like bugs:
1) Mirror Golem doesn't attack (ignores) some hostile creatures:
- The Watcher (that one like Sphinx, with wings) in the King's Forest crypt and a few other locations in Anthology.
- Freak of Ages (mini boss in ROT3 C18)
Even if they have some immunity/resistance/absorption for physical damage, Golem still has elemental damage, so theoretically could hit that things.
2) Sometimes after the defeating Belthezar (ROT3 C18 final boss), the pirate captain doesn't appear, so nothing happens... PC can't take Nether scroll, there is no exit in area and the only option is to quit/re-run via Return stone. Probability of this bug is about 40%-50% (4-6 of 10 runs).
3) In the chapter "Anthology C6 Luskan" there were Harrad's Trackers that easily killed my Sorc at first time. I resurrected in a Temple and then went to ROT3 C18, where during the dialog with Jard, the same Harrad's Tracker suddenly appeared and almost killed my char
Further - more, after dialog finished and Ruins of Trinity map loaded, I clicked "Rest" and guess what? That bastard appeared again and attack my char 
This time I killed it, but the fact that Harrad's Tracker could track me down through time and space is incredible... is it intended so? 
By the way, this Tracker is a real challenge for Sorc. Mirror Golem and Epic Solar don't last long (need to be called back), and Lvl 6-9 spells hit only 1 of 3-4 times (at that, all puristic feats are taken). So it takes a few minutes for my Lvl 30 ER3 Sorc to kill only one Tracker... and of course with a help of "Bigby's Crushing Hand" for permanent holding this monster... May be add to this NPC the same description as one of King Devastation Beetle ("Run far away") ?
4) Weapon of War (C6 Luskan final boss) periodically deals a constant damage from distance (Electric + Sound + something else) despite of successful save throws. In the battle logs I see: deals 107 + 107 + 107 damage no matter of result of saving throw (for example, Reflex vs Electricity). Is it by design? So the full damage can't be avoided?
5) not a bug, but a fun thing:
In most cases the "Luskan commoners" are not so "commoners" - actually they are quite powerful mages (on an equal basis with Arcane guys?). Their naming and their look are misleading
but I liked that, it was a surprise for me to be suddenly smashed by commoners