Small overclock fried card?

Using MSI Afterburner I slightly overclocked a R9 270x
(1080>1150) (1400>1450) (stock voltage)
I put a stress test on it and it would not go over 60 degrees, so seemed fine.

I then ran a game on it for the first time, the system restarted and now the card has permanent horizontal and thick vertical lines across the screen (even in the bios/at boot.)

I swapped it out for an exact same model card (which I overclocked to the same amount) but that doesn't have any issues at all.

My question is, why would one card by fine with an overclock and the other not?
Or, maybe I'm thinking it was just a coincidence, and one of the cards was already bad?

Thanks.

The damage was not from overclock. The card was defective right from the start. If its under warranty, RMA the card and get a new one.

ok thanks for the input. Will send it back.