The Rotten Apple bonus round rule is one of the many rules that can be applied during the Bonus Round episodes. It can only appear on floor 3 for any character.

Rotten Apple

Maximum health no longer increases when you level up.

When this rule activates, the maximum health will no longer increase on a level up. The text will state: “But your maximum health doesn't increase…”

This does not prevent health increases from Dramatic Exit, and your health will still be refilled when you level up.