Not so long ago, I tried watching through all the Highlander movies. Each sequel seemed to take the first film, ignore some elements of it (like, you know, the ending
) and continue from there. None of the sequels were at all consitant with each other. It's probably the worst continuity in movie history.
Recently I saw Ghost Rider: Spirit of Vengeance, and was a little suprised that it ignored the first Ghost Rider movie, even going so far as to flash back to a different version of the scene where he signs his soul to the devil in order to save his father. Nicholas Cage was still Johnny Blaze, but played the part differently.
Then there's X-Men: First Class and Wolverine: Origins, both prequels to the X-Men trilogy yet making several changes along the way (in fact, I recall reading an interview where the makers of XM: FC cited JJ Abrams' Star Trek as their example of putting making a good film ahead of slavishly adhering to continuity)