Continuity mistake: When they jump in the pool, they drop the guitar. When they jump out their guitar is no longer in the pool.
Coco (2017)
Directed by: Lee Unkrich, Adrian Molina
Starring: Benjamin Bratt, Gael Garcia Bernal, Edward James Olmos, Alanna Ubach
Other mistake: When Miguel is singing "Un poco loco" on stage, he goes out of range of the mic, but his voice is amplified.
Continuity mistake: Miguel is thrown into the pit and lands in water, being completely submerged. But a couple minutes later when Hector mentions Coco, Miguel pulls the family picture out of his pocket, and the picture is totally dry and showing no effects of being submerged.
Trivia: The number A-113 can be seen in Ernesto de la Cruz's album, often referenced by animators.
Clerk: I am terribly allergic.
Miguel: But Dante doesn't have any hair.
Clerk: And I don't have a nose, and yet, here we are.
Héctor: What are you doing?
Miguel: I'm walking like a skeleton.
Héctor: No, skeletons don't walk like that.
Miguel: That's how you walk.
Héctor: No, I don't.
Ernesto de la Cruz: I have to sing. It's not just in me. It is me.
Question: Why didn't the aunts just tell Miguel that Ernesto De La Cruz wasn't his grandfather when Miguel was rambling about it when he entered the Land of the Dead?
Answer: The aunts may not actually know. Because his ancestor hated him so much, she may have talked about him very little in both the real world and the afterlife.
Answer: Because Miguel didn't unambiguously talk about his wrong assumption. In other words, he never said, "my grand-grandfather and Imelda's husband, Ernesto de la Cruz" (which is wrong). He only called him grand-grandfather and Imelda simply referred to him as a good-for-nothing musician. The aunts, uncles and the grandfather Julio all knew about Hector, as can be inferred from the reunion scene.
Question: After Miguel comes back to the land of the living, does the family believe that he went to the Land of the Dead? And if not, then do they believe that Hector got murdered, since the letters to Coco prove that he was the writer of De la Cruz's songs? And if they believe, do they manage to prove this to the general public and that is why they hate him? Or do they just hate him because they discovered that he lied about writing his own songs?
Answer: It is never actually addressed what Miguel told his family about his adventure, so it can't be said whether or not they believe him. It would seem to make sense that he would tell them though, it was a pretty fantastic tale after all and I'm sure he would want to tell his family about it. It would also make sense that his family would believe his story, it's really the only way he could have known that Remember Me was a lullaby Hector used to sing to Coco. The letters Coco saved prove that Hector was the actual writer of De la Cruz's songs and it was later proven to the general public, hence the graffiti on De la Cruz's memorial and a memorial for Hector taking it's rightful place in town. It can also be reasonably assumed that the general public is aware that De la Cruz murdered Hector since the theft of the music and Hector's death go hand in hand. It wouldn't make much sense for people to hate De la Cruz so much if all they thought he did was take credit for his partner's music after his partner was already dead.
Answer: It is never really explained in the movie, but we could assume that whenever someone dies they keep whatever items they had on them. Hector may have had the photo on his person when he died.