PaRappa the Rapper co-creator Rodney Greenblat has said in response to a fan email that, if he were to do a new Um Jammer Lammy game, he would have Lammy and Katy Kat as a romantic couple. A few years after this, he would begin posting annual Valentines' Day images to his Twitter accounts with Lammy and Katy together.

Notably, one of the interviews in Um Jammer Lammy Now! has Katy laugh off a question about if she has a boyfriend.

In the original Japanese and European versions, Paul Chuck's song had a line about "chopping trees down for fun". In the US, this was changed to "Knowing that we're here for the fun" in Lammy's song, and changed two times due to being repeated in PaRappa's version: "Rockin' to the beat just for fun" and "Rollin' up high in the sun". The censored version is used in Um Jammer Lammy NOW.

Stage 1 of Um Jammer Lammy was incomplete in the original PlayStation version, not being accessible in either multiplayer or PaRappa's story. With a GameShark code, a US player can access an unfinished version of PaRappa's stage 1, which has no animations and only uses triangle buttons. Stage 1 would eventually be finished for Um Jammer Lammy NOW!, released 9 months after the PlayStation version, with Rammy and PaRappa support.

Stage 1 is the only PaRappa stage to use the instrumental of the Lammy version.

There are two versions of the 6th stage. In the Japanese and PAL versions of the game, the stage takes place in hell. In the American version, the stage instead takes place on a volcanic island. Both stages have different graphics for Lammy, a different intro cutscene, and a slight change in lyrics in the first stage to remove any references to hell or the devil. The Japan-exclusive Um Jammer Lammy NOW! retains the island plot from the US release.