What's the best plot twist you pulled on your players?

I am currently dming an Achtung! Cthulhu campaign, and one of the villains is Captain Wolfgang Reiter of the Nachtwolfe. He has shown that he will cooperate with the characters if it furthers his own goals (he very much doesn't want the Black Sun to reach their goal) but he's still a nazi, so he will betray the characters with no qualms.

This is exactly what he did. I sent the characters on a mission to retrieve two magic objects that the Nachtwolfe want to give to an American millionaire who wants to use them for his presidential campaign, and Reiter was the courier. This millionair has loaned a lot of cash to the nazi party before the war, and the magic items are his repayment.

The American courier was the son (Lewis) of said millionaire, who went to Switzerland to retrieve the items.

Unbeknownst to all of them, a regular spy of the German secret service was there to kidnap Lewis in order to make his father forgive the loan (she has no idea that there are magic items involved, or that magic exists at all).

In all this, Reiter convinced them that the regular spy was the courier, that she worked for the nachwolfe, and that he'd help them, they never even thought to ask if he was lying or not, they trusted him blindly. This devolved in a very satisfying standoff and later a car chase, and now they hate the guy with a passion.