Death Nail = Death Knell

This one’s easy to mix up, because it’s actually a cross between two correct phrases (though correctly separate) with almost identical metaphorical meanings.

“That’s a Death Nail in this team’s chances.” The allusion being a final nail hammered into a coffin before burial.

“That’s the Death Knell on that idea.” The allusion being to the old world pealing of church bells when someone has passed away.

In short, both allude to something being over, or whose time has passed. But in the interest of full disclosure, the term, Death Knell, is the older originator of the term, Death Nail.