This is likely to be a 'deliberate error by the film-makers', but in one scene you see two characters putting up a poster advertising a gig. The date shown is Saturday 23rd December (year unspecified). The nearest year that this date fell on was 1978, which is 2 years before the film was released. It is possible that various anachronisms in the film may occur due to this, for example release dates of records, or registration number plates of new cars.