https://www.theregister.co.uk/2018/11/06/apple_watch4_511_update/

It isn't just Microsoft that has QA issues – so does Apple. The Cupertino giant withdrew a watchOS update that bricked the Apple Watch 4 last week, and has now rushed out a replacement containing things that don't work yet which Apple probably didn't want you to see.

5.1.1 supersedes the catastrophic 5.1 update that sent Watches into a boot loop. There isn't an easy user-activated reset (a "Ctrl-Alt-Del" equivalent) for this situation, and some Apple Stores are offering owners replacements.