And yea, if the client is allowing credentials to expire while a file is still open, that's a problem.

Since users acquiring credentials,  the credentials getting put into the kernel context,  and the
user opening files are all asynchronous activities, Its likely that some of the time the issue
will not be due to bad user behavior ( leaving files open for wide periods of time ), it will
sometimes be simply a file being opened too close to credentials going away

Frank