Harry Potter and the Philosopher's Stone/Harry Potter and the Sorcerer's Stone - Day of the Week Error
Excuse errors in detail, I'm doing this from memory, as I can't locate my copy of the book. In HP1, Dudley comments that it will be Monday, and he'll miss a show. Following from the date of Sir Nicholas's 500th deathday, this is July 31, 1991.
91+22+31+0=144. 144/7=20 with a remainder of 4, indicating that July 31, 1991 was actually a Wednesday!
Copy of the book