These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Teriyaki Yoko. Especially when you fail and she rises out of the ground, 100 times her size. Even in the American version.
There's something about the sunset scene Stage 5's cutscene that's very spooky and unsettling... The color error and animation could be suspect.
Scrappy Level: The fifth level in the original (thanks to a sudden Difficulty Spike in having to out-rap every previous Master) and the sixth level in Lammy (while not as difficult to play by itself, some people find the background music for the level to be simply god-awful).
Stop Helping Me!: Boxy Boy's tutorials in PaRappa 2, which interrupt cutscenes and can't even be turned off for the experienced players.
That is, unless you press the start button.
That One Boss: Potentially, any of the bosses, but Cheap Cheap the Cooking Chicken is consistently hard for a lot of people.
That One Level: The bathroom rap level is considered to be extremely frustrating due to how many buttons the player has to play back with near perfect timing in order to succeed. This was probably one of the reasons why the sequel has a more forgiving mechanic for button pressing in the levels.
Big Lipped Alligator Moment: After Lammy has eaten too much pizza and tries to run but collapses, a parade of expectant bunny parents led by Cathy Piller marches out of nowhere and sings a brief version of "Treasure", which describes that Babies Make Everything Better! This gets us all like "WTH?"
That One Level: Many players have the toughest time with Cathy Piller or Teriyaki Yoko. The former gets lampshaded in the game.