The code may well be the clearest way to communicate the subtlety. That doesn't mean a comment isn't helpful to communicate that there's subtlety there and give some idea of what sort of case it's meant to be about.

  // It may look as if you could just say p->adjust() here,
  // but that doesn't work because of a subtlety involving
  // cancelled credit cards belonging to purchasers in Uganda.
  // Please tread carefully!

... And then, if talking to another human being is the only real way to grasp what's going on, you use your version control system to find out who added that comment and go talk to them.