#1 - April 12, 2012, 6:26 a.m.
I just did a cathedral run and got the exact same glitch in the D3 beta (twice within a few minutes). First I noticed that when I cast spells, no spell graphic appeared (it turns out the spells were being cast from the location where the server thought my character was, not where my character appeared to be on my screen). Then I ran into the edge of the world. I ran back to the location where I desynched (which I found by looking at where my skills were being created), and everything went back to normal (just how it works in D2).
Also, in Diablo 2 it tended to happen in areas where your character got stuck on a wall or something (because the server would miss the moment where you walk around said object), but both times it happened to me were in pretty open-ish areas, as far as dungeons are concerned. The second time it actually happened during or immediately after the SK cut scene.