365 days of SL, day 140:
(Click for larger — 1920x1033 PNG, 811 KiB)
Genuine graphical glitches are pretty rare in SL — you may get objects blinking in and out of existence if (presumably) your video card can't cope with the complexity of a scene, and of course you may get misrenderings if your viewer doesn't support certain features like mesh (go upgrade your viewer, BTW!), but other than that, graphical glitches (as opposed to non-graphical glitches) are a rare occurrence.
I got the above last night, though, while camming around and homing in on a piece of skybox building. The effect was relatively stable while it lasted (i.e., I could move the camera further without destroying it), but after it disappeared eventually, I didn't manage to reproduce it.
Who knows what was at fault here, though. SL? Firestorm? My video card? Its driver? There's really too many layers of complexity involved to even make a guess.