i have this problem quite often.
i noticed it most frequently appens if you have your material applyed on a geometry touching other geometry with different material. i find that grouping geometry by material reduce the occurance of this bug.
i also find the same problem sometimes using the "duplicate material" option.
so recently i try to avoid it, because when i save and reopen the file, sometimes the materials which were duplicated disappear and are replaced with the "parent" material (the one from wich you started the duplicate one).
i noticed this bug more often occours on material which have no texture maps applied.
another similar funny pain in the ass is the case in which displacement/bump map are corrupted (i mean, you notice the displacment bump fails and when you go to check the bumpmap folder the jpeg texture lose the prewiew in windows and can't be opened by windows picture manager) this is veeeery fun indeed.
i find the more safe way is to pack/save wathever .vismat you create and always save the .psd which you use for bumpmaps because you may need it again when vray "eats" bumpmaps.
i really hope this will be solved with next relase.. and really hope it will be relased very soon. i do love vray itself, and the new features are so promising, but i hope they deal with bugs at first.