Interesting. This prompted me to look at my Mac and it's running 10.10.3, I never got a prompt to update to 10.10.4 or 10.10.5, but when I open App Store it tells me there's an upgrade to 10.10.5. I guess Apple managed to break the automatic update mechanism in 10.10.3.

I wonder if this is related to the behavior where my iMac wakes up every minute starting every morning at 2AM. This is so obnoxious that I now turn my iMac off at night instead of putting it to sleep.