How do hydraulic flat tappet lifters fail?

Here are the lifters that are currently out. You’ll see the three on the far left have some marking. It looks worse in the picture, I can BARELY feel the marks with my fingernail. I can’t remember which one, but one of those is #7. The #7 pushrod looked just fine. 2B40FC8A-A422-46D1-A040-FF51B25AB039.jpeg30539BD0-BFB0-4626-A958-ACC3DCF4679C.jpeg