>Why wouldn't an ON ERROR be triggered by the "invalid seek offset" error message?

Most likely because it's considered an internal error, which "should not happen", or which is not caused by VFP. You write that you usually get this error during the night, which brings back memories. It's not unusual to set up a machine to do a lot of things during the night, or some other "idle" time

1. Have you checked whether you have some "power saving" features enabled, which stops your HD, or sets your NIC in "power saving mode"?
2. Automatic backup routine?
3. Automatic update?
4. Automatic HD defragmentation?

Personally I have a web server which used to occasionally hang mysteriously during the night. After a loooong time I found out what happened. I have two servers, one application server running IIS and WWC, and one data server. Both servers ran Windows 2000 Server, and were set up to update autoamtically at 03:00 AM. The servers were perfectly synched, but the data server had much more RAM, and took a few seconds longer to boot than the program server. This ment that when the program server started it's applications, the data server was not available yet, and the applications reported that it could not find their data, in a messagebox....! As soon as I turned of the automatic update, the problem vanished.