Look at this hydraulic SBC lifter

[HAFROD]

This is from my Salons new 350 Chevy. I always moly lube and use Comp break in oil and add their zinc additive also. Broke it in for 20 minutes as I always do. I had a seaping head gasket so I removed the heads. I decided to pull one lifter at a time and inspect. Found this one that has a circle grooved into it. It catches my finger nail. This motor has probably an hour on it. Have not had it on the highway cause of no front clip or windshield. I have seen a lot of dished lifters from flat cams but never a circle cut into one in the center. What do you guys think?

 

[lilbowtie]

It took something awfully hard to do that - I would guess the cam lobe has some kind of imperfection to do that as the lifter spins. I would have a good look @ that lobe, you can even get your pinky in there and feel it all the way around. My guess!

 

[Injectedcutty]

I might be wrong, but it looks as if the lifter somewhat collapsed causing it not to spin on the cam lobe which wore the crown of the lifter past "flat". It's a longshot, but there might also be an issue with the valve/pushrod/rocker on that particular spot. As lilbowtie said, maybe see if you can get your pinkie down onto the cam lobe to see what it feels like.
I would definitely be tearing into it to make sure nothing else is damaged. Hope you find the culprit and get back on track!

 

[565bbchevy]

My first thought was a burr on that lobe since the lifter is obviously spinning in the bore as it should and doesn't appear to have any other abnormal wear, a picture with a machined straight edge going across the bottom would also help.