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When ATMs were first implemented they would dispense money before or at the same time as giving your card back.

This resulted in premature conclusion errors. They went to the ATM to get money. They got their money so they left... forgetting their card.

When ATMs were updated they fixed the design error. Now the card popped out and had to be removed before money would dispense. This resulted in a spike of people leaving with money hanging out of the ATM because they had been trained that removing their card was the end of the task. (Which is why money now gets sucked back into the machine if it hasn't been removed fast enough).

The point of the anecdote (other than that you hire HCI experts before implementing an interface) is that implementing a new and objectively better system doesn't necessarily result in an objectively better outcome when replacing an incumbent worse system.