What errata or house rules help balance occasionally-broken scenarios?
That's pretty much how the game is designed. It is a catch-22; sure, it's completely "random" and new every time, but that also means it's impossible to predict and balance accordingly.

Something me and my friends do when these "broken" haunts happen is tweak the conditions around a bit. An example would be haunts that require exorcisms: If we are all high sanity/knowledge characters, we may add 1 or 2 to the number of successes needed, or subtract them if we are dumb/insane. It helps to create an even playing field at the start of the haunt, but still leaves opportunity for game-changers as the game goes on.