I've found this phrase across the internet and everywhere they use "A user interface" instead of "An user interface".
Is it an error that has been propagated or is it correct? If it's correct, why?
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It's because the use of a or an is based on the pronunciation, not the spelling. Since usage is pronounced with the y sound (which does not count as a vowel), most people would put a before it. It goes both ways, though. The h in honor is silent, so it would be an honor. Many years ago, any word that started with an h would have an before it. For example, an humble home. That isn't common in modern times, though.