Also in the "well reviewed, but you've probably never heard there was a sequel," category are Ip Man 2 (there's a 3 being advertised on Amazon, but it's either a fake or a bad bootleg... hard to tell from the reviews... but 2 is real and the first was a good enough film to make me consider buying the sequel), Full Metal Alchemist: The Sacred Star of Milos (which is a film based on the series, which is itself a reboot of a previous series which had its own film... so sequel?!) and the Alec Guinness version of Tinker, Tailor, Soldier, Spy, which I've somehow never seen.
These things just manage to sneak up on me, but sometimes it gets way out of control, and not always in a good way. Such is the case with Highlander which has had recent (as in the last 5 years) releases of an animated movie, Highlander - The Search for Vengeance and a live-action, direct-to-TV-and-then-video Highlander: The Source. The latter, being one of my top contenders for worst film of all time, right up there with Misery Brothers.