These days, movies are scrutinised like never before and phenomenon that is Harry Potter is no exception. Eight core films and two spin-offs (to date) have probably been viewed more than any other movies since the Boy Who Lived made his screen debut in 2001 and eagle eyed viewers have spotted more than a few mistakes within the franchise.
Hardcore Potter fans have long criticised the movies for leaving out entire story arcs such as Hermione's house elf protection initiative: S.P.E.W. and for leaving characters out completely such as the late Rik Mayall's role as Peeves but these aren't the mistakes we're looking for.
Instead, the mistakes/gaffs/bloopers/errors in this case centre on continuity, ignorant deviation from the source material and accidents that probably could have been avoided.
Some mistakes have to be forgiven purely because of the convenience factor all movies take advantage of. It couldn't be helped that Daniel Radcliffe sported blue eyes and brown hair where the Harry Potter detailed in the novels had bottle green eyes and jet black hair nor can you really criticise them for casting Emma Watson as Hermione even though the young actress had near perfect teeth and Hermione in the books most certainly did not.
However, that isn't to say that all the convenience mistakes will be ignored because some of them are just so ridiculous that it would be unfair to leave them out.