Factual error: Prior to the Yom Kippur sermon where Rabbi Stiller is asking for forgiveness for not revealing his relationship, the cantor sing the Kol Nidrei prayer, which is only sung once a year on Yom Kippur eve. Totally obvious mistake to anyone who knows anything about Judaism. Clearly Stiller and the script writers do not.
Factual error: During the multiple hospital scenes, the doctors direct their updates to Will, ignoring the closest family member, her grandmother. Will's relationship with Charlotte is not even a year old. No hospitals or doctors would ever discuss this condition with anyone outside the immediate family, including and especially the Cleveland Clinic.