Early on, South Park (1997) creators Trey Parker and Matt Stone were lined up to write the prequel for $1.5 million. They later decided to not work on the project and returned all of the money to New Line.
Due to events that retroactively occur/are stated in Dumb and Dumber To (2014), When Harry Met Lloyd is no longer canon.
The truck that pulls in front of Jessica as she is chasing the short school bus is hauling "Turbo Lax", the same laxative Lloyd gives Harry in the original Dumb and Dumber (1994).
Lin Shaye was the only cast member to star in both the original Dumb and Dumber (1994) and the prequel.
Eric Christian Olsen was repeatedly denied the role of Lloyd because the producers thought he was "too good-looking". It was only after seeing some home-video footage of Eric in full make-up improvising a scene with already-cast Derek Richardson that they gave him the role.