Lifter Failure

Most likely was poor machining on the lifter face or maybe a tight bore that didn't allow the lifter to rotate correctly.
I just put in a set of Comp SFT lifters that I got five years ago and they needed to be refaced before they were used. Luckily @NC Engine Builder did the motor, saw that things weren't right, is setup to properly grind lifter faces, and he corrected them before anything went sideways. As a matter of fact, they wouldn't rotated correctly in freshly bushed lifter bores, so they were truly FUBAR. But to the naked eye with a straightedge, they looked OK. Don't take anything for granted with valvetrain parts these days. Check and re-check.