The following contains major spoilers for Coco. Go enjoy the film, then come back once you've seen it.
It seemed clear going into Coco exactly what the story of the movie was, however, as it turns out, there's a bit more going on that there initially seems. The story follows Miguel, a boy whose family has forsaken music, who discovers through an old photograph that his great-great-grandfather was apparently Ernesto de la Cruz, the world's greatest musician. When Miguel finds himself in the land of the dead, he goes looking for his distant relative. Although, if you've seen Coco you know that's not the whole story, but the truth was in front of you for almost the entire film.
Miguel comes to believe in his relationship to Ernesto de la Cruz because of a photograph. His great-great-grandmother had ripped her husband's face off the picture after he had left the family never to return. However, after accidentally breaking the frame, Miguel discovers a part of the photo that had been folded back, out of sight. The man in the photo is holding a guitar, and it's a very special guitar. It's the one Ernesto de la Cruz was always seen playing. On all of his album covers, in all of his movies, he always uses the same guitar, with a skeleton face drawn into the headstock. Miguel is such a fan, that he draws the same head on his own guitar, seen in the image above. We eventually see the actual guitar later, and the design is the same.
The truth, however, is that Ernesto de la Cruz is not Miguel's relative. In fact, it turns out that de la Cruz actually killed the man whose guitar it was and took it, along with all that man's songs, and used them to become famous. In reality, it turns out that Hector, the nearly forgotten man who agrees to help Miguel early in the movie is actually the great-great-grandfather Miguel was looking for.
It's not the most shocking twist in the world, and many adults in the audience may have figured it out on their own anyway, but some might have noticed the way the solution was actually given very early on. If you look at the grin of the skeleton head in the guitar, you'll notice that one of the teeth is painted gold. It's an odd affectation to add, unless of course, you have a thing for gold teeth. As it turns out that one character in Coco also has a single gold tooth, Hector. The guitar was actually Hector's, which you realize once you discover this similarity.
I ended up seeing Coco twice and only picked up on this on the second go round. I was on a specific hunt for easter eggs, and thus was probably more inclined to notice it. No attention is ever brought to the guitar design, even after the revelation of Miguel and Hector's relationship is learned, so most people probably missed it. Even on the second viewing, it's a great thing to discover, as Pixar hid the truth in front of you the whole time.