This is one article in the larger Troubleshooting SharePoint Alerts guide
Task lists can send an email notification when a task is assigned or changed (Settings > Advanced Settings > E-Mail Notification).
If you modify the Assigned To column of the task list to “Allow multiple selections” then the email may be sent to random, incorrect recipients – some names will be crossed out (strike-through) in the email.
“On a SharePoint Server 2007 site, you modify a task in a task list. When you do this, users other than the users who are assigned to the task receive the notification e-mail message for the change.”
This fix is believed to have been included in