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.
No doubt they later realize they'd have been better off accepting his offer.
Complete Monster: Dr.JosefHeiter was a leading authority on separating conjoined twins before deciding to try the "reverse" of this. To that effect, he kidnaps a truck driver and two American girls in order to use them as test subjects against their will for the "human centipede," which consists in joining the victims, mouth to anus, causing unimaginable suffering. However, he kills the truck driver due to him not having the same matching DNA as the two girls, and then replaces him with a Japanese tourist. When one of the girls almost escapes, Heiter punishes her by making her the "middle" piece so she's forced to feel the same pain twice. When two detectives come to Heiter's home to question him about the suspected disappearances, Heiter tries to drug them to add them to his centipede with one of the girls getting sick. When it doesn't work he simply tries to murder them. Heiter is just a perverted psychopath of a mad scientist willing to subjecting his victims to unthinkable horror in order to pursue his insane and selfish goals.
Dancing Bear: Some people don't care about the quality of the movie, only how ridiculous it is. Felix Clay in Cracked's 6 Horror Mashups Just Crazy Enough to Be Awesome calls it a "Shlock shock horror movie that was made solely for the sensationalism of saying it was actually made [...] The novelty of this movie ended with the idea of it."
The Centipede in general has been parodied almost to this point. A common game on the internet after the film came out was to decide who should be in a real-life version of the Centipede, and what positions they should be in. Centipedes made from hated celebrities (Justin Bieber, Paris Hilton, various Disney Channel stars) are among the most commonly proposed.
Katsuro's ridiculously poetic threat to Heiter: "The Japanese possess unbelievable strength when backed into a corner!"
Heiter's really pathetic attempt to mimic chicken clucking.
"SWAAAAAAAAAAALLOW IT, BITCH!!"
Then there is the actual Human Centipede itself. It's just three people kneeling with their mouths sewn to their asses. The concept seemed much scarier in your head now, doesn't it?
The last 20 minutes or so of the sequel zigzags between this and Squick. It's really hard not to laugh when you see all the victims sewn together because you know this physically can't happen. But at the same time, it's really hard not to gag when they all shit in each other's mouths. But then Martin pukes afterwards, and the next thing you know, you're laughing again. But then the pregnant woman turns out to be Not Quite Dead, gives birth in a car, and crushes her baby's head underneath the gas pedal. Start gagging again. But THEN Yennie hits Martin in the balls, and shoves a centipede up his ass while he pathetically whines. Startlaughingagain.