When opening a shared workbook, I receive the following message:

"Unexpected file lock by {username}.  You can override this lock or continue
without doing so.  If you continue without overriding this lock, you will not
be able to save this file, but you can use File Save As to save your work to
a different file.  Do you want to override this lock?"

However, the workbook is not truly open by anyone at the time this message
is received.  I have verified this by inspecting the PC of the {username}
specified as well as reviewing the Open Files listing on the network.

What causes this to happen, and how can it be corrected or prevented?