I just found a seemingly-cosmetic-but-actually-gameplay-effecting bug:
Playing a Halfling Transcendent Fist (just to clarify that that troll is definitely not my character

. What appears to happen is that an actor will move into a square at the same time as I do, with the end result that we are both in the square (with the player sprite hidden). Moving out to an adjacent square, the player sprite becomes visible again. (this is how I determined which square the player was considered to be in.). If the enemy moves out instead (via the player waiting), the player sprite does not reappear, and in fact the enemy continues to target the player, but any hits are inflicted on the enemy itself.
The positioning effect is also odd, it appears to attach the player to the monster, and when the player steps out from where the monster is, it gives the bizarre message 'Forest troll's clone steps out from hiding'.
It's rather exploitable, especially if all enemies experience this mistargeting of their attacks (haven't checked this yet).
The red box indicates the square I was in on the previous turn, with the red arrow indicating where I moved to.
So far, this has occurred twice in Trollmire: once with a white mouse (probably a clone), and once with this troll, which I can verify is a clone.
EDIT: Actually, the circumstances in which this occurs may be different from what I thought. I've recently triggered it twice by killing a clone (with no other NPC around)
EDIT2: Also, is it possible to disable cloning in town areas? Unless you're already being exploitative (drowning NPCs), there seems to be no benefit or interest to it, just slowdown.
EDIT3: This bug is actually extremely exploitable, because I realized that you are unhittable in this state, but you can hit others without losing it; you just have to not move from your current square. Functional invincibility.
I'm getting the impression that some clones may also pull the same disappearing act after landing a hit on me, but this is only an impression.
General impressions:
* Pretty grindy, no surprise there.
* For a melee-er, bosses seem often easier, actually, because you can pick to some extent between engaging a clone or the original. With good tactical positioning, this means that the boss can only use smite-targeted attacks, or attacks that harmlessly pass allies. If this seems too easy, perhaps give the original the ability to 'push past' clones (much as the player can swap places with townsfolk). I'm looking at trying an Alchemist, which might be fairly broken (large AoE damage that the player is immune to, when the cloning system generates plenty of crowds... yeah.)
* Makes levelling substantially easier. Not sure whether this is a plus or minus. I was running around dungeons kind of out of order and didn't really notice any problems.