The old classic of two enemies being interrupted by someone they can agree is worse. Not just approaching “the enemy of my enemy” but it happening right there and then. Love this one. Particularly when the new alliance is uneasy. They may become friendly rivals later, of course.
Borrowing a partial quote from issue 7 of Marvel’s GI Joe, one of the first examples I ever saw of this trope, where our heroes team up with the Oktober Guard against Cobra. The Guard are an immediately likable bunch, which helps sell it. (I think the Imperial commandos teaming up with Luke and Leia against a weird alien threat in a Star Wars comic came first. This seemed more of a stretch, as Imperials are way worse than Russians.)
A hunter pursues a decent-Humanity vampire into an office building, with a small night staff there. Then the Sabbat/Strix/VII show up. Can the vampire persuade the hunter to protect the people inside, avoid breaching the Masquerade, fight the worse-things-than-her and not get a shotgun blast to the back of the neck at an inopportune moment?