The scene where Joel stumbles upon the memory of his visit with the doctor, even though there are two Joels, there are no special effects being applied. As the shot turned back and forth, Jim Carrey actually dashed behind the camera, switching from a standing up wearing a jacket and stocking cap, to sitting down wearing no jacket no hat. It's actually quite impressive.

There is a carnival scene in the movie which actually took place in Times Square in New York. The cast and crew had to run around after it to get a perfect shot.

In the scene where they're looking at the elephants, director Michel Gondry pulled Kate away from Jim - so in the shot where Joel looks around for Clementine, it's actually Jim Carrey's real reaction, because he's looking for Kate Winslet.

One of the original ideas for the ending was to show Clementine walking into the Lucana office, showing that Joel and Clementine did, in time, learn to hate each other, but the film makers decided it was best to let people draw their own conclusions.