Corrected entry: When Frollo shoves Quasimodo's mother backward on the steps of Notre Dame, she falls, hits her heads on a step, and dies. However, if she hit the ground hard enough to get killed, her head would have been cracked open and there would have been blood all over the place. When the Deacon comes out of the church and discovers her, he even chastises Frollo by saying (singing) "See there, the innocent blood you have spilt." The problem? No blood is visible at all. This is due to the fact that Disney will not show evidence of violence in their films, but it is still a mistake. It just doesn't feel right when the characters can see the blood and the audience can't.
Corrected entry: Phoebus claims he has been called home from the war. Problem is, France wasn't at war in 1482, and hadn't been since 1451.
Corrected entry: When Quasimodo's mother is running from Frollo, she stops at the Cathedral to beg sanctuary. She turns to see Frollo coming after her. When she does she is standing very close to the cathedral. When she turns to run again, she is instantly several feet away from the cathedral wall with no time to get there.
Corrected entry: In the film's opening it is stated that Frollo gave Quasimodo his name out of cruelty, and that it means "half full". Actually Quasimodo translates (roughly) into "in the manner of". In the book the name was given to the character because Frollo found him on Quasimodo Sunday.
Corrected entry: When Frollo locks the family inside the mill before setting fire to it, he bolts the door by using a spear as a cross bar on the outside. The door opens in and would not be blocked by the spear.
Corrected entry: At the end of the song "God help the outcasts", Esmeralda bathes in the sunlight of the rose window. Seen from above, the projected light forms a circle on the church-floor. As the rose window is circular itself, and the church-floor is perpendicular to the church-wall, this is only possible when the sun is at exactly 45° above the horizon (the projection would be an ellipse otherwise). However, the Feast of Fools was held at the end of December or the beginning of January, and in this specific case on January 6 1482, according to the original manuscript of Victor Hugo. The highest point the sun reaches in Paris around that day is maximally 20°, even when taking care of the Gregorian Calendar Reform in 1582. Also, the rose window is not exactly southward, which means the sun will have even lower altitude when shining straight through it. Thus, there's no way the sunlight could form a circle on the church-floor around that day. (00:38:00)
Corrected entry: In the scene that Frollo is speaking to Phoebus about the dregs of Paris and illustrating it by lifting a stone and revealing all those bugs underneath. He slams the stone back down crushing the bugs, but the stone was turned upside-down, even though the whole scene never loses sight of Frollo holding it, and he never turns it that way.