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.
Critical Research Failure: The film claims that Jack and Jill are "identical" twin siblings, for the sake of having Adam Sandler's face on both of them. Identical twins are made out from a single fertilized egg cell, which results on a single gender offspring. Fraternal twins are from separately fertilized egg cells, resulting in differently-looking twins or boy-girl twins.
Actually this was lampshaded when someone asks if they are identical or fraternal. However, it is possible but very rare to have Semi-Identicle twins, in which case they would be fraternal but they developed from the same fertillized egg that were fertillized with two different sperm
However, the female usually turns out underdeveloped and with Turner Syndrome, so it's still massively farfetched.
Dude, Not Funny!: Even though it was CGI, seeing the poor hamster squirm while taped to Jack's son's back was just cruel.
Family-Unfriendly Aesop: Remember, folks, if you have an obnoxious relative who talks too loud, says inappropriate things, and always makes themselves the center of attention, you're a Bad Person(™) for not wanting them around.
Unintentionally Unsympathetic: It's clear that whenever Jack yells at Jill we're supposed to be siding with Jill, but she's often very rude and nasty to everyone she meets, and is generally a public nuisance, like when she takes a call while watching a film and then complains about how the people trying to shut her up are being so loud she can't hear the phone. She also invades upon every aspect of Jack's life (and has been invading upon every aspect of his life FOR YEARS), even though it's clear he doesn't want her there, even extending her stay over many more weeks without asking Jack.