HP Inconsistencies from book to book
In OotP Harry wonders about Sirius watching him play Quidditch, and wonders if Sirius thought he played as well as his father. The line in OotP is "Harry had never asked him" BUT in PoA Sirius tells Harry that except for his incursions into Hogwarts to try to get Pettigrew the only time he went out was to "watch the quidditch" and Sirius TELLS Harry "You play as good as your father did, Harry" Harry never asked him because he didn't have to, Sirius had already told him he played as well as his father.
In SS Quirrel attempts to kill Harry by strangling him, resulting in Quirrel's death. Why didn't Quirrel simply "zap" him with the "killing curse"?
I know there are more but I can't think of any at the moment.