Factual error: When Cruella as a child makes her way to London and crosses the zebra/pedestrian crossing, there are visible zig zag markings. These however were not introduced until 1971 in England, which is the time period when Cruella is older.
Revealing mistake: Right after baby Estella is born there is no umbilical cord.
Character mistake: Whenever Catherine visits the Baroness with young Estella, before she gets murdered, she asks the Baroness for money so that her and Estella could start a new life. Since Estella is being spoken about, the Baroness is aware of her at this point. However, later on in the movie, the Baroness questions John, "What did you think I meant when I said take care of the problem?!" Implying she thought Estella had already been killed when she was a baby.