The slip-up occurs during the collage sequence when Frida and Diego Rivera first go to New York aka Gringo-landia. The slip-up is, in fact, during the chapter called "The Invasion of Gringo-landia". When you are looking at the collage, look at the picture that has Diego facing left and Frida being connected to the back of his head facing right. You can clearly see that Frida (Salma Hayek) has no unibrow in this picture, and she has one during the rest of the movie.