And the ratings will drop below the cancellation threshold over half a season?
No, if anything something like this will cause the ratings to go up especially if they air the episode. However, the problem in the U.S is when things are successful and something goes wrong management, rather than risk a lawsuit, usually settles with the people involved and axes the program to prevent further lawsuits.
I'm sure the people involved in the mythbusters' accident have lawsuits pending right now against the Discovery Channel for public endangerment, destruction of property, etc. While I don't think it's enough to cancel the program I have a feeling there won't be anymore explosive myths.