Also been tied to random other occurrences, ...
It's happening fairly frequently here in our classrooms. Haven't bothered to try to figure out what sequence of events causes the behavior but I know it has been several different things.
It's really hard to track down because it's such an obscure item... it's very much in the peripheral site, for me, at least... and I never know it's gone until I go to hit it... well if I check the command log all the way back to the last time I hit it... there could be 1000s of lines gone by, by then! So it's rather hard to track.
Hopefully it goes away in 2009, although it IS little more than a nuisance.
BTW, Just got my Subscription Center login up and running, and am checking out some AU screencasts and materials... saw that you had some resources for Autocad solid modeling. I look forward to checking that out.