In the book Rule of Two Darth Bane's apprentice, Zannah, meets a follower of the dark side named Hetton who has 8 Sith assassins working for him, modest dark side abilities of his own, and who is rich in Sith artifacts and knowledge acquired by wealth.
Zannah brings the 8 assassins and Hetton to face her master. The novel says that she could not predict who would win the battle.
Bane defeated Hetton and the assassins, then attacked Zannah. Zannah got Bane to stop attacking her (in part) by explaining that she had not turned against him, but that she was delivering Hetton and his guards to be slain by Bane. When asked, she said that if the guards and Hetton had killed Bane then that would've meant he was weak and deserved to die. The book says Bane realized that her actions were in "perfect accordance" with the Rule of Two and his other teachings.
But isn't this the exact situation which both the Rule of Two and the Brotherhood of Darkness were meant to prevent from degenerating the strength of the Sith? Bane had explained that prior to the Brotherhood the Sith had become weak because many weaker Sith would gang up to defeat a stronger master. In this way the Sith became weaker and weaker instead of stronger and stronger.
At this point in the series Zannah was weaker than Bane, who still had his Orbalisk armor. Hetton was weaker than Zannah and the average assassin guard was presumably the weakest of the group. So, if Hetton and the assassins had killed Bane wouldn't that have made the Sith weaker and wasn't Zannah therefore breaking the Rule of Two?