Other mistake: Harry now has Dudley's toy room and Dudley's is next to his, however when there is a shot outside the house there is only the one window on the left side of the house and not much space between it and the left outside wall, where is the window for Dudley's very narrow room? (00:09:00)
Stupidity: Harry learns the owner of the diary from the writing on the back and it 'introducing itself'. It's plausible that the name doesn't alarm him; he grew up in the Muggle world. It does imply however, that since learning of Voldemort in his first year, he has not attempted to find out Voldemort's real name/identity. Similarly, Ginny engaged with it, even though she grew up in the Wizarding world and presumably would know the name. Or she never looked at the back or 'introduced herself' to the book.