In Encounter at Farpoint, Data - in his first meeting with Riker - that he was a graduate of the 'Starfleet Class of '79'. This is generally assuming to be an error, or 'Characterisation Marches On'. However, I have sometimes wondered what Data's character might have been like if he'd been in Starfleet service for nearly seventy years when signed on board the E-D (probably as Chief Science Officer, rather than Ops Manager as a Bridge position that should either be a junior officer's position, possibly similar to Worf's role in s1), rather than only about twenty years (and presumedly mostly restricted to 'below the decks' roles that didn't allow much interaction with the crew by fanon and licensed media).