I've never had this bug kill my systems, but it can make Bryce unstable. It is easily avoidable, if you regularly export (and then .ZIP up the files) your mat libraries! One thing, too, is that Bryce doesn't really delete objects form the libraries. They are still there, in terms of disk space used. The way around this bug is to hold down SHIFT or CTRL, and select all of the objects you WANT to keep, export them, replace the User file with a clean one, and then reimport your models... Sounds tedious, but it's not so bad! (not half as bad as keeping track of poser files!)