Continuity: In Quidditch Through the Ages, it says that the first ever World Cup was in 1473, with a match being held every 4 years. Yet the World Cup in 1994 is number 422. There can't be 422 world cups in 521 years with a four-year gap in between. Calculated correctly, the World Cup that the Weasleys, Hermione, and Harry attend is actually the 131st, and it should have been held in 1993.

Factual error: In Hogsmeade, Sirius implies that at a certain point, Snape's attendance at Hogwarts overlapped with Bellatrix Lestrange's. But given their dates of birth, Bellatrix would have left school two years before Snape started.

Continuity: When Fred and George bet on the Quidditch match with Bagman, it says Fred put the paper with their wages in his robes, but Fred was wearing Muggle clothing.