The additional form fields and the master login fields overwrite each other sometimes. For example, say you add the extra question answers to your lastpass vault entry for your bank login. When you come back later, your password has been replaced by the name of your second pet, or some street you lived on. When this happens, the password that got replaced is not recorded in password history.

If you must use the Lastpass extension, beware the wrench icon under 'Advanced Settings'. It might just lock you out of your bank if you use it.