bent valve and lifter damage

Managed to bend an exhaust valve last weekend and I'm having trouble tracking down the cause. The damage I have is a bent valve, a lift with scuffing on the sides and the lifter bore with a groove about 5thou deep and about half an inch wide toward the bottom of the bore. The cam is all good however thelifter was hard to get out probably because of the scuffing. The piston has a kiss on it but not worth worryig about.
I'm guessing the lifter got hang in the bore and held the valve for long enough for the piston hit. But what would cause the lifter scuffing?
Forgot to mention this was a flat tappet cam. Cast iron block.

 

Lifter

check and see if the lifter spread or forked on the roller end. Don

 

Maybe a foreign object was floating in the oil and got lodged inbetween lifter and bore.
Are the other lifter bores wore to out of spec? This will also eat up the lifter quickly.