When Harry and Lloyd are in the car, Lloyd asks if Harry wants to hear the most annoying sound in the world. This was not originally in the script (you can tell by the look on Jeff Daniels' face).
Jim Carrey chipped his tooth years earlier, but had the cap removed for the film to make his character look more deranged.
Jeff Daniels later claimed three of his agents desperately called him the night before he was to have his costume fitting, begging him not to go, saying they would get him out of his contract. They told him in no uncertain terms he was making the worst mistake of his career, that he would destroy his reputation as a serious actor, while Jim Carrey would receive all the credit for the movie's comedy. Daniels told them his decision was final, and if it was a mistake it was his to make.
When the film went to the top of the US box office, it gave Jim Carrey the distinction of being the first actor to have three films go straight to number one in one year (those films being this, The Mask (1994), and Ace Ventura: Pet Detective (1994)).
Following the release of this movie, Jim Carrey and Lauren Holly did get married in real life, but the marriage only lasted a year.
When film critic Roger Ebert went to see the film, he wasn't very positive about the movie as a whole, but he did admit that he laughed so loudly that he embarrassed himself during the scene where Billy the blind boy is seen stroking Harry's dead pet bird Petey. Ebert added that he did not laugh as loudly again, or very often, which was fine because "if the whole movie had been as funny as that moment, I would have required hospitalization."
Jeff Daniels won his first Emmy award the night before production began on Dumb and Dumber To.
Trivia items from IMDB
Listen to my conversation with "Dumb and Dumber" screenwriter Bennett Yellin HERE
