Us by Jordan Peele. It’s a great movie, but that ending scene is terrible. They don’t need to insist so much to make sure that the dumb viewer gets it… It’s heavily hinted during the last few scenes, and letting the viewer wondering alongside the terrified son who his mother really his, and who we’d like her to be, would be so much more impactfull.

Also Rec by Jaume Balaguerò and Paco Plaza for similar reasons. The movie spends it’s entirerity building this unseen menace, and establishing a terrifying ambiance, and then for some reasons right at the end, just before they let us alone with our thoughts after the movie, decides they should undo it all and give us the “oh actually it’s just this guy who summoned this demon” ending.