The Succubus (a.k.a. Chef's Mama) - S3-E3
Stupidity: When Kenny and Cartman are sitting on the bench, Cartman can't start the tape as he has fallen asleep. Kenny then tries to wake him up, but wouldn't it have made far more sense for him to just start it himself, as he is sitting right next to it? Cartman's job was to push the play button. It was not like he had to rewind, fast forward or even eject the tape, so all Kenny had to do was just simply push the play button and tape would have started to play.
It's Not Easy Being Green - S5-E16
Stupidity: Unless the tow truck was someone else's or Hank had the keys surely Dale and Bill could've used it to drive Boomhauer's car back.
Young Kidaichi's Murder: File 1 - S2-E1
Stupidity: Kindaichi behaves in a criminally illogical way in this fourth parter episode. The original victim told 4 separate people some sort of keyword for the party, each one linking to another person. Kindaichi after at least the second if not the first victim could and should have asked EVERYONE (he had every opportunity for that) if they received such a 'password' by Tachibana before he died. Instead, he pointlessly follows the trail of breadcrumbs and two if not three people die for nothing.