Just recently I've had a couple of weird experiences where I've walked towards a monster and ended up with both of us occupying the same tile. It didn't happen at all when I went right through to Dredmor with a melee build, but now that I'm playing a wizard it's happened at least 2 or 3 times and I'm only up to level 2. It seems to be happening when the monster is in a DoT field like the flames from a fireball, although I'll have to see if I can consistently reproduce it.
I'm not sure what's causing it, but I've also been experiencing this on a Windows 7 64bit version of the game through Steam.
There are some movement/turn timing issues left to clean up, this sounds like it falls into it. Fun fact: for the longest time you could actually move without spending a turn (therefore monsters not moving) with an exploit to this bug.
Your build has nothing to do with this bug. I've experiencing it several times in Windows 7 - 64 bits and Windows XP 32 bits too. It happens when you try to move before your turn arrives. I have the bad habit of moving before I get the turn when several monsters are acting, so sometimes when I've hit a key towards an empty space and a monster moves there before me, it's possible than instead of hitting him, my character moves on top of the monster. I think it could be avoided if you wait your turn THEN move. I'm working on my patience... :-p
