Apple has determined that, under very rare circumstances, a crack may form along the rounded edge of the screen in aluminum models of an Apple Watch Series 2 or Series 3. The crack may begin on one side and then may continue around the screen as shown in the images below.

This is from the page that was just put up on Apple’s support site.

If the screen of your Apple Watch is cracked, go to this page to see if it is eligible for a free repair.