When Tita receives the roses from Pedro for being the ranch's cook for one year, they are light pink/coral. When she's preparing to make quails with rose petal sauce, the roses are red. This is a nod to the novel upon which this film is based, wherein it is explained that the pink roses turn red after their thorns scratch Tita's chest (as shown in the film), and the petals absorb some of Tita's blood.
When Dr. Brown holds up a piece of white phosphorus, nothing happens. White phosphorus ignites in air at room temperature, however, so it should have been shown burning.
At Esperanza's wedding to Alex Brown, the groom is shown opening a bottle of André Champagne, complete with plastic cork, in 1934 long before this product was available.
Background music while Tita and Nacha are cooking in the kitchen tells the story of a car breakdown.
When the narrator says "that night she [Tita] cried and knitted" Tita is clearly shown crocheting with one hook instead of knitting, which requires two.
Reflected in a window during a dolly shot as Tita leaves the table while everyone eats their cake.