SharePoint 2013 HTTP Error 500 and 503

At some point after a reboot / iisreset or something else, I found that I could no longer get to my SharePoint sites nor to Central Administration. Each page was returning HTTP Error 500 or 503.

Of course, any time this happens, you need to take a look at IIS Admin. So opening it up, I see that all my application pools were offline. That's strange, it was just working a moment ago. Opening up the Basic Settings, I see this:

For some reason the Start application pool immediately box was unchecked. I then proceeded to check all of them and restarted my application pools and everything now works fine.

Weird.