Many of us have the alternate adsense ads page installed but hardly ever look in our logs to see if it's being displayed or if it is, what pages are invoking it.
Ran into a situation today where I found Google must've tweaked something and I didn't notice that these pages were no longer serving AdSense, I don't see any stop words on some of the pages so no clue what's wrong yet.
However, I came up with the idea of the AdSense alarm to immediately alert me to this situation.
Instead of pointing AdSense to a static alt_adsense_ads.html page, I decided to change it to a dynamic page so I could send emails to my mailbox notifying me when the ads aren't displaying.
When you have multiple ad units you don't want to be notified when a single ad unit doesn't display, this can happen at any time. Therefore, using a session cookie I counted the number of times the alt_adsense_ads.html page is displayed per page. When it counts 3 ad units not being displayed for a single page (see HTTP_REFERER for the page), it will now notify the webmaster that AdSense has stopped on that page.
There you have it, and AdSense alarm!