I usually have a process that I go through before I import:
-Check the exports to see if they are OK
-Gather user info for the schema that will be imported
-Once I have all the info and a "gaurantee" that I have enough to import, I drop the user(cascade)- re-create the user giving him the exact tablespaces, grants and permissions that he had before the process.
I always get errors if I don't do this carefully. Don't drop anything unless/before your sure of your exports and backups.