So you are in a gunfight, your Lancer is with you and the bad guys are surrounding you, you are about to be overwhelmed by then... wait, what's that thing moving behind you inside your hiding spot? Why, it's that... Guy! He opened a hole from beneath to give you guys a chance to escape! Obviously, you take it, and both you and the Lancer follow him to the sewers to escape... Everything's good so far... But then, you receive a phone call. It's your informant! He must have a crucial piece of information! Take up the call and what you hear is... "Are you with Guy?! You are?! Listen to me, run away as fast as you can!"

You are surprised by this but before you can tell your buddy, you look up and see that Guy is stabbing the Lancer... or hell, before you can react, he already stabbed YOU.

That's basically the trope.