[eluser]JHackamack[/eluser]
It depends on the alert. I have the following conditions set up
People reach a 404 page (using the 404 error page I set it up to email me)
When somebody puts in the wrong information, or information that doesn't match I run a cron job every night and populate an alerts table (which is then displayed in a view to look at later)
If you want realtime alerts on if something isn't write one way to do it is create a "watcher" model that is called after you do a database insert to check. However depending on the alert you can always return the form back to the user saying the information is incorrect.
I wouldn't suggest a background monitor unless you have your own server as some hosts frown (aka will disable) that kind of behavior and i haven't found many hosts who allow triggers (then again i might not be looking that hard)