Originally Posted by Mueslix
A coaster brake has a lock ring? Someone school me if I'm wrong, but I thought the only way a coaster brake could have an actual brake failure is if the little strap somehow broke or got loose.
Sprockets ("cogs") on coaster brakes are kept in place with a "snap ring" - a springy steel ring that snaps on the hub very tightly, immediately after the sprocket, keeping it on. It's a very reliable locking mechanism, I have no clue how that can fail. Those snap-rings are damn hard to remove or put back on.
The only way it could fail is, if it was rusted like heck.