Odd Issue with File Poller

Does anyone know what events trigger an IS file polling port to move a file into its error directory rather than the completion directory? I’m experiencing some weird behavior with a service that receives an XML node from the file poller (content type set to text/xml) and calls queryXMLNode to get data that is written to a log file. The service completes successfully, the log entry is written, no exceptions are thrown and the file goes into the error directory. If I programmatically throw an exception as the last step of my Try block, the file gets put in the completion directory. I’m running IS 6.1. Thanks,