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.

When asked where he got the inspiration for the game's surreal plotlines, Gabin Ito, the chief writer and editor of Um Jammer Lammy, stated that the team of the game did not do drugs, but rather got their ideas from a combination of a lack of sleep making them spacey and a limited development time not providing the time-frame to fine-tune concepts.

The cutscene for the final stage in Um Jammer Lammy has Lammy, Ma-San, and Katy Kat all rush to the same door and apologize for being late as they got in a "lot of mess", implying that Katy and Ma-San had similar adventures to Lammy. In the American version of the game, Lammy gets to the door in her camouflage outfit from the censored island stage, and to adjust for this, Ma-San gains a Middle Eastern turban and camel, while Katy gains a military helmet and bullet strap, giving a further clue as to what adventures they could've gone on.

The European version of Um Jammer Lammy and the US version of Dino Crisis mistakenly use the Japanese version of the generic PlayStation anti-piracy screen.

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.

Um Jammer Lammy character designer Rodney Greenblatt expressed disappointment in the US print marketing campaign for Um Jammer Lammy, which highlights the midriff of a live-action human Lammy and has copy text describing how players "might even score groupies out of this", particularly due to what he perceived as a double standard in how this commercial was allowed in the US, but the level in which Lammy goes to Hell had to be censored into an island for that same demographic. He would say and reiterate in an interview:


The game's co-creator Masaya Matsuura had a more muted response to the ad, calling it "a little too sexy".

Lammy's design went through four different variations that were approved and then rejected until it reached the final design influenced by the Australian pop star Natalie Imbruglia. Character designer Rodney Greenblat was "a little disappointed" that the final design strayed so far from looking like a lamb.

Rodney Greenblat was cautious about Um Jammer Lammy not featuring PaRappa as the main character and using rock music instead of lyrical performances. He also suggested basing the game on goth aesthetics, such as those of the band The Cure, but this idea did not go through.

One of the background characters in the "Fire Fire!" stage of Um Jammer Lammy, a blonde-haired and beige-furred firefighter dog with a crop top, is shown in various PaRappa the Rapper spin-off media as being PaRappa's mother.

The design of Lammy's shirt in Um Jammer Lammy is based on Stinkabod Lamé, a character from PaRappa series character designer Rodney Greenblatt's Apple Pippin interactive storybook, Dazzeloids.

In 2022, Rodney Greenblat uploaded a VHS rip of the earliest beta versions of the game's cutscenes to YouTube. These cutscenes feature very little music, placeholder voice acting by Ryu Watabe, and an early pinker character design for Lammy which Greenblat thought made the character appear more lamb-like, but was rejected in favor of the final red-haired design.

This VHS rip also notably features two unused cutscenes that were changed significantly in the final game:

• An early intro cutscene showing the rest of MilkCan already backstage and waiting for Lammy to arrive, featuring the Jet Baby movie playing on TV rather than in a theatre and an advertisement for MilkCan's show that night. This cutscene also shows Sunny Funny, PaRappa the Rapper's crush, as a member of MilkCan, seemingly a backstage presence as she is absent from the MilkCan poster shown. This ties in to the 100% completion bonus from PaRappa the Rapper, which has Katy Kat and Sunny Funny performing in a band together.
• A silent cutscene that appears to be an early version of the Joe Chin Museum Ma-san cutscene. In this cutscene, she is seen reading in a library rather than a museum, specifically a magazine featuring two disco dancers about to kiss. Ma-san, presumably irritated by the male dancer who appears to be the only one talking before this, begins drumming rapidly on a reading desk. Her drumming causes the library to shake, making books fall off the shelves and causing the male dancer to have a profuse nose bleed as they are kissing. The female dancer slaps him away, causing another stream of blood to shoot out of his right nostril, before cutting to a smiling Ma-san.

Additionally, some small differences from cutscenes used in the final game can be observed:

• In the opening of Stage 5 in the final game, Lammy accidentally trips over a skateboard. In the prototype version, she intentionally steals it.
• In the final game, Lammy uses objects as guitars whenever she sees arbitrarily placed imagery relating to gambling, uttering the catchphrase "The guitar is in my mind! Leave it to Lammy!" In the prototype version, she doesn't need prompting to imagine a guitar, and says the catchphrase "I have no choice left... leave it to Lammy!"
• The fake credits of in Stage 6's cutscene feature actual staff credits instead of the game characters.

In the opening cutscene to Stage 1, the excuse Lammy comes up with while running backstage is an exact description of the setup to the Stage 5 song "Full Tank" from PaRappa the Rapper, ending on a cameo by the teachers from that game and reusing voice clips of them saying "Let's rap for it!!" from Full Tank's opening cutscene:

At the beginning of Stage 5, during the intro when Lammy is panicking, she will make a face similar to that of "The Scream", a painting by Edvard Munchs.

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.