The Hacker's Guide says that if you are using VFP3 :
-->In VFP 3, when you use table buffering, you have to either commit or revert changes before you can close the form. Otherwise, you get an error message—fair enough. What isn't fair is that when you're using a private data session, the form goes ahead and closes and leaves you with a data session labeled "Unknown" with the offending table still open. In fact, anything that prevents a table from closing (like a trigger failing) when you close the form causes the same problem. The moral is that you must be sure to either commit or revert changes as part of a form close. If you should get caught in this situation, you can go to the View window, choose the Unknown data session, select the open table, issue TableRevert(.T.), then close the table. Once you've closed every table in the Unknown data session (any relation to the Unknown Soldier?), the session goes away.
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No doubt you aren't using such an old Fox, so it must be something else...

-LTS