Cutting yourself because you caught a dropped knife is a mistake. Breaking an arm because you braked too hard and got rear-ended in traffic is a mistake. Planning, anticipating, and undergoing an elective surgery and suffering from the intended result is not a mistake, it's a decision.
Maybe I'm unsympathetic, but when I hear "mistake," I think of something I did not intend or know the consequences of. If I were to take medicine solely used to induce vomiting and I was not only aware of its effect, but actually wanted it to do so, I don't claim it was an accident when I throw up and feel miserable. If you do something intentionally and the expected unfolds, that is a choice and you're not a victim no matter how much you don't like it.
They said mistake, not accident. A mistake can be intentional. An accident cannot. I can make a mistake while doing a math problem, for instance.