[James2001]

The 1990 Only Fools and Horses Christmas special is itself basically an extended first episode of the 7th series, as it sets up the storyline for the rest of the series. Yet on both DVDs and repeats, it's treated as a standalone episode, and gets skipped over during showings of the regular episodes in order, even though the episode doesn't really stand alone, unlike the other Xmas episodes, and going from series 6 to series 7 without showing that episode creates massive plot holes (in fact you need to have seen the 88 and 89 specials to properly understand the 1990 one and series 7).

Otherwise you go straight from Rodney and Cassandra's wedding to Rodney split up from Cassandra sleeping on Del's sofa with Raquel moved in with no explanation (indeed not even knowing who Raquel is if you haven't seen the 88, 89 and 90 Christmas episodes).