It should come as no surprise that Hollywood is really good at recycling movie ideas. You know it, I know it, and we expect that we’re going to see things we’ve already seen before, countless times. That, or they’ll turn a one-off film into a franchise and try to beat it into the ground, and they’re obvious about it.
But not always. Sometimes, they release a sequel to a film, but no one knows that it is one. These films are either official, or unofficial sequels that weren’t widely advertised as such. Whether that harmed their success or helped it, is up to you to decide.