Continuity mistake: It is mentioned in dialog that during the last bird attack (the first film), a red rain was visible that seemed to drive the birds mad. However, at no time during the original film is there any red rain visible.
Other mistake: Throughout the film, you can frequently see many people's faces blurred, phone numbers blurred, store-fronts blurred, etc. This is because the production could not afford the rights to show these people, places and things, and simply blurred them out.
The opening scene was supposed to be a reference to "Saturday Night Fever", but the production could not afford the proper soundtrack, and thus, Bill "strutting" down the street for a prolonged period now lacks any frame of reference.