We came across an error the other day where we were saving an Excel spreadsheet across the network and a service on the server, utilising the FileSystemWatcher, would then process the spreadsheet. The error we were seeing was a sharing violation from Excel and a processing error from the service.
It turns out that when Excel saves a document it creates it first then fills it. So the FileSystemWatcher was waking up and trying to process the document before Excel had finished saving it. This caused the service to fail, as the document was empty, and it caused the sharing violation in Excel as Excel couldn't finish writing the document as the service had opened it. :(
The work around was to save the file locally first and then copy it across the the server.