Broken Lifter Retainer

Well since the only way to tell is by pulling the intake, half of us could have a broken tab on the retainer and not know about it. Someone messed with it as mentioned above, or worst case you've got something funky going on with one of those lifters (or the valve / pushrod / rocker / spring) that put some vibration into the keeper and made it self-destruct. Just a wild guess.