Alice and Bob are excited to see the new adaption of one of their favorite TV shows,books,video games or whatever. But when they finally saw the movie...they thought it was a waste of time and money. Wait isn't the main charcter a little kid, why is he in his twenties? Where the heck did this romance come from, this wasn't in the original? Why does the villain look nothing like he did in the original?! Where did this plot point come from?! I thought we were saving the world from an alien invasion, what do these magical macguffins have to do with it!!!
Obviously in an adaption, something's have to be changed, but when you change too way too much from the source material, your movie could be suffering from Michael Bay Adaption Syndrome. This condition describes a movie that changes so much of the contuinty from the original source material, the fans not only think it's an unfaithful adaption ,but that it feels like a completely different project. There are several causes for this from something simple like trying to fit an entire series into a 90-minute film or extreme Executive Meddling from the studios to make it Darker and Edgier, often resulting in a rating outside the source material's target demographic.
While the trope namer was Michael Bay's infamous live action Transformers movie, the trope dates way back to the 80's and 90's, maybe even further in the Way Back Machine.