Sharing violation on an NT domain

We have an NT domain. Occasionally users try to open a document and get a “Sharing violation” warning message saying that X user has the document open, but they have not had it open for some time. Document can stay in this state for some hours or days but eventually will unlock itself. We make a copy of the document so the users can carry on working. Users are on W95.

So why would a document that has not been used for some hours still thinks it is being used?

Thanks for you help Michael