If your program is running on a workstation and the data is at a file server, i found out that the following cases could produce an Illegal Seek Offset.

Your program is running in a multiuser environment maybe on a LAN but could also be a WAN. It's possible that on a given moment the OS disconnects the DBF file connections with the server. The DBF file on the workstation is still open but when the workstation request some more data from the DBF (some data usually is cached) it can produce an Illegal seek offset.

This might be the case if you are using the suspend mode of Windows 95. a Customer of my had the same problem. Simply turn of the suspend mode and the problem should be gone.

There are also a few registry setting at the server that have some influence on disconneting file connections

If you are working with a WAN it could be possible that the wan connection fails on the moment you're requesting more data of a open table.

Some other error messages related to this problem are:
-Cannot access selected table.
-File read error.
-File write error.
-Transaction in progress.
-Invalid file descriptor.
-File not open.