'Friday the 13th: The Final Chapter'
(1984)

After grossing $33 million on a $2.6 million budget, the Friday the 13th producers decided that the fourth, and supposedly final, film in the franchise would NOT be the last. They put out a fifth film the very next year entitled A New Beginning. Convenient. And they didn't stop there. Paramount made eight more movies (and counting) after The Final Chapter.