The Elder Sign. The designer gave an answer to this in an earlier thread:
If you are on a city sealed by the Elder Sign, you cannot add Cultists to those locations willingly. Therefore, you would need to suffer Atlach-Nacha's insanity loss instead. If you are at 1 Sanity or Insane already, then it would simply have no effect.
Re: Atlach-Nacha and Elder sign, Reporter's insane ability