I have an unusual problem with User Information List in my main site collection. After unknown circumstances, we lost quite a few users from that list (their profiles still exist). Moreover in .../sites/ collection, list is workin properly.
What're the consequences? In collection i can "use" that user, e.g. add him to permissions list or pick in peoplepicker. However in my main site collection, there's no such user and i cant change or use anything related to that user.