The phenomenon that occurs when you realize you've been conflating two or more different actors or actresses as a single person. Most common with young actors before they make it big or character actors.
Shia Labeouf and Topher Grace
Kurt Russell and Patrick Swayze
Rachel Bilson and Mila Kunis
Omar Epps and Mekhi Phifer and Sean Patrick Thomas
I had serious diactorism this morning when I realized Shia Labeouf wasn't on That 70s Show. Turns out Topher Grace is a different guy.