It's happened on TV shows and movies like Bewitched, The Dark Knight Trilogy, That '70s Show, Pretty Little Liars, Shameless and Roseanne. It rarely goes unnoticed but occasionally happens due to trying to find the right actor, scheduling, disputes and even in some circumstances; death.
What is it?
And this time, it's heading for Gotham.
Gotham, the story of Detective James Gordon before Batman, has just finished its second season and when the show returns for its third, one of it's characters will be a little bit different.
Ivy Pepper, seen in nine episodes across the first two seasons, had originally been played by Clare Foley, but now producers are looking for an older looking teenager to take over the role, which is now being bumped up to a series regular role.
"Casting is underway for an actress in her late teens to assume the now-series regular role of the would-be eco-terrorist."
Ivy Pepper is based on the DC Comics character of Pamela Isley (aka Poison Ivy), an eco-terrorist with an obsession with the environment, who uses plant toxins and pheromones for her nefarious activities as well as having a deadly touch.
News has also come through that Gotham season three will see the introduction of Vicki Vale, who was most notably played by Kim Basinger in the original Batman film, and Dr. Jervis Tetch (aka Mad Hatter), who will be making his first live action appearance since the 1960s when he starred in the Adam West TV show, Batman.
Any ideas on who could play the new Poison Ivy? Will you be watching Gotham Season Three?