Small details that matter the most. I didn't even think of the UX impact validating each input field has until compared to validating all fields at once. I just imagined myself rushing to complete registration and being constantly disturbed a couple of times.
It's important to do it at the right time! As i mentioned, once the user is done and tries to click the button, the validation could happen as it's just in time before the feedback comes that he actually submitted the form.
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