@kiwidude & @user_none: I've run into a consistent problem with some of the books uploaded to the Library here on MobileRead. An example is:
Sherlock Holmes Omnibus. If I only download the .mobi version, the Count Pages plugin fails with an error "1050 Failed To Count Pages". If I download the ePub version, it counts them. (Are there really 2191 pages in that omnibus?! Impressive, HarryT.)
If I do a mobi to mobi conversion, it will now count them (and the count is pretty close to the ePub count at 2046 pages.)