I'm having this problem and I don't know how to go above this.

In your MOSS 2007 or WSS 3.0 implementation, you have a document library with
checkin / checkout enabled. If a user with read-only ("Visitor") permissions
looks at the document options from the drop-down menu on the document name, they
have no option to check in or check out.

However, if the same user opens a document from that library, the document will
open as read-only in its MS Office application (Word, Excel, etc.), but the user
will see the "Server Document" bar with the message "To modify this document,
you must check it out" and the "check out" button. Once i do that, i let me
change but would not let me check-in. I then closed the document and re-open it
again, this time it let me check-in with the new changes. Why is this happen
when the group call "Visitor" have READ ONLY permission. Does anyone know how
to solve this problem?