Costner opted to pass on Shawshank to work on a personal project that ended up flopping massively (see, 1995's Waterworld). Tim Robbins ended up taking the role, and the rest is history.
Michael B. Jordan, Straight Outta Compton
Everybody loves Michael B. Jordan, especially Dr. Dre. So, when Jordan was approached by Dre to star in the musical biopic Straight Outta Compton, he expected him to jump at the opportunity. Unfortunately, Jordan had to pass due to scheduling conflicts because he was actively shooting Fantastic Four at the time.
At the end of the day, Fantastic Four ended up being a massive failure, and Straight Outta Compton received countless accolades, and more than tripled Fantastic Four's profit.
Jim Carrey, Elf
Jim Carrey was actually locked and loaded, ready to play Buddy the Elf. But, massive production changes, budget cuts, and a recession forced the movie's production schedule back 10 whole years. By then, Carrey had already taken the role of the Grinch in How the Grinch Stole Christmas.