fairly often that breaks the OAB. If an admin modifies the smtp address of a user object
to something non valid (like inluding an ~ character) the generation of the OAB will
fail until this is corrected. The only way to catch this is to turn up diagnostic logging
on the OAB and then rebuild the offline address book. In the app log you should see the
errors that will point to the offending object in AD.
Checkout the article for a ton of great info on other stuff that can break the OAB: