Well mainly because it doesn't really leave you with an argument.
I said your example does not constitute a mistake. I don't recall you asking me for examples that would constitute a mistake.
And it is not something that does no harm. A mistake may or may not cause harm (just as reckless behavior might or might not). A mistake is an error that is not intended and does not represent inattention to or violation of rules, policy, law, etc.
Some examples:
A nurse is working in an ICU. They are ordered to give a medication. They get paged while they are preparing a medication and forget that they already administered a patient's heparin. They give them heparin again. This is a mistake.
A nurse is working in an ICU. The system is designed to give drugs one at a time to prevent errors by requiring that they scan the patient's wristband after each administration, and that's the official policy. This is tedious, so, instead, every nurse in the ICU when they have multiple medications to administer scans the med then the wristband for all the medicines at once and then administers all the meds. This is at-risk behavior.
A nurse is working in an ICU. The same system is in place. The nurse thinks the system is bullshit. The nurse works around it by taking the patient's wristband off and bringing it to the med room and doing all the scanning in the med room. Then the nurse keeps all the medication vials with them throughout the day and administers the meds without checking. This is reckless.