When: 2003

Moving from one co-star to the next, Ashton Kutch met Amy Smart while filming The Butterfly Effect. Smart caught notoriety for her role in Road Trip, and her career was just starting to pick up steam. Although she was object of Kutcher's affection, there was one small detail that prohibited the would-be from playing kiss and tell: Smart was already in a longterm relationship with actor Branden Williams. Nonetheless, speculation of a possible hookup persisted through the movie's January 2004 release, but died down soon thereafter. 

Smash Probability: Co-stars fraternizing while filming a movie is a time-honored Hollywood tradition. Smart had a boyfriend at the time, so anything that may have taken place was effectively kept under wraps.