Trying to determine if there are any other known issues that would cause user alerts to be deactivated automatically other than the documented:

"When an alert generates an excessive number of results, the portal site deactivates the alert to conserve its resources. You can edit or delete this alert from your My Alerts page."

The user claims this is not the case and a review of the recent changes of the list she is subscribed to appears to back that up. This particular portal is set for 30 alerts per user at 20 results each. I myself have received alert notifications with 20 results each and the alert was not deactivated.

Or is this just another random Sharepoint mystery?