Sudden Application pool shutdown / 503 service unavailable
Today I suddenly had our web-site shutting down on us. It gave me a Http Error 503 while it all worked fine a minute earlier.
I checked IIS (IIS7 in our case) and I saw the Application pool had shut down. We couldn't understand how the application pool could just shutdown. This could potentially shutdown hundreds of sites in one go.
Apparently there is an option in IIS that will shutdown an application pool after 5 errors within 5 minutes, called Rapid-Fail Protection.
Disable this feature and your application pool will keep on running.