What software were you viewing it in, and what format?
Possibly if the file structure of the models was change either a newer version or older version of say DirectX(.x), and then loaded into a modeling program you might end up with a little scrap that the program doesn't know what to do with.
Example:
The model was made 2 years ago by Anonymous and saved in the DX8 format. 2 years later, you download it and open it with your program that imports the newest DX format, DX9. See what I'm thinking?
Of course, I'm assuming most models here are .mdl and an updated .mdl format is not likely to be the problem.
I'll be interested to find out what the problem is.