What's the one thing you've learned about pricing that no framework told you?

An unlimited plan was offered at $99 a month because simple pricing felt fair. One user processed over 50,000 API requests a day and cost $1,200 a month in cloud bills. When asked to move to a usage based tier they were furious: "The plan says unlimited." We were paying $1,100 a month for the privilege of having them as a customer. What's the pricing lesson you only learned after a customer broke the model you designed?