Corrected entry: It is often mentioned that Janet and Jonny were together at school, yet in "When Janet Met Jonny" they supposedly meet for the first time in a nightclub.
Corrected entry: Donna's younger sister seems to disappear after the first season - and it's not mentioned what happens to her after Donna's Mum dies.
Correction: More than likely she is living with her dad - it was mentioned in series 6 that Donna's dad is also looking after her chav-like brother.
Correction: "When Janet Met Jonny" at the end of series 3 was one of two "special" spin-off episodes that may well not be canon with the rest of the series. The other "special" episode was at the end of series 6, in which all the main characters died, yet they were all alive and well for series 7. If the events of "When Janet Met Jonny" were referenced in any other episode then this may be a valid mistake, but as it is, the episode can be considered non-canon.