The Google Reader redesign fiasco? Google Wave to Google+ transition?
It's not like Apple "usually reverts in quality" either. This was a business decision, probably forced. They had to create something of their own for maps from what they could license and buy quickly.
Goes to show that you should not depend on your competitors for core technology. Which Apple always tries to avoid, but probably in 2007 (when the iPhone come out) the didn't think of Google as a competitor. They weren't in the phone business back then...