I was running and jumping in Vivec (with my new Orc) and, on two separate occasions, fell through the bridge separating the Foreign Quarter from the Redoran district. Any idea why this happened, how I could avoid this, and how I could reproduce this?

That's because Morrowind has crap collision detection which bugs out on occassion. I've never found a way to fix it, not even code patch helped. Good news is that this only seems to happen on structures, not on terrain, and in exteriors - which means it'll be the most noticeable in Vivec.

All I can really suggest is to wait for OpenMW to be finished and look for entirely new, glorious ways to break it :-P

The bug will most likely be reproducible in OpenMW, it uses original game files so any quirks in the meshes ought to persist. I understand there is a morrowind overhaul of Skyrim in the works that will be using new meshes and textures, but I don't know if anyone has plans to replicate that work for OpenMW.

Post 1.0 for OpenMW the developers are considering fixing bugs that stand in the original (though they'd like to avoid "fixing" bugs people are particularly fond of), but that's at least a couple of releases away in my estimation. Even then, if it's an error in the terrain mesh as I suspect an existing mesh improvement mod may already address the issue. Thirty seconds of searching didn't turn up any likely suspects, though.

I had a different glitch the other day. In town, I cast a levitate spell on a cliff racer overhead and it fell, and then the game froze. Note that I had just loaded the game so it could not have been an out-of-memory issue.

I reloaded, tried again, and the game didn't freeze.

Hi,
i've had the same thing happen to me just outside Balmora.
Only solution afaik is to not jump on the bridge at all.

If you cast a weak levitate spell on a cliff racer, it will lose the ability to fly, and will fall down, taking falling damage when it lands. If the cliff racer falls far enough, it will die.

Of course, there are other things you can do to them, like damaging their stats (0 strength + a bit of encumbrance = can't move, for example, and some cliff racers carry racer plumes which have a low, but non-zero, weight).

I really love the Spellmaker, especially since it was what got me interested in the series in the first place (and why I might not get Skyrim even if it gets a DRM-free release). Of course, it hasn't always been exactly what I would call "balanced", but it's fun, and that's what matters, right? Games are, in general, meant to be fun.

I have now tested this quite a lot of times and it appears to be caused by saving while jumping (while in the air). It haven't happened to me otherwise.

I am pretty sure I wasn't saving in mid-air when I had this happen.