Google has been utilizing the Developer Preview system for the past several major updates of Android. This gives Android the ability to find and squash bugs prior to an official release, but more importantly it allows developers to develop their apps to run on the new version of Android. Google will be the first to admit that the Developer Preview builds are Beta and as such will have bugs. They suggest running the stable versions if you don't want buggy software.
One pesky bug has popped up over the past several days. Nexus 5X and Xperia Z3 devices running the Dev Prev 4 build have been finding themselves in an endless reboot loop when wifi is enabled. The bug has been acknowledged by Google who have already said a fix will be present in the next update.
via code.google.com