I would like to know if it ever happen to native speakers that they use A before vowel sounds. I mean, are they always accurate? Does it ever happen that after using an you decide to use some adjective which does not start with a vowel sound? How natural is it to use an before vowel sounds?
The use of “an” is based on pronunciation, not on spelling. “An” is usually used before words that begin with a vowel, but a better “rule” would be that “an” is used before words that sound like they begin with a vowel.
I’d say native speakers make very few errors with “an” when speaking. Using the word “a” where the word “an” should be is just to difficult to pronounce when speaking at a normal speed. IF an error is made, it would most likely be in writing — either through simple carelessness or because the person was focused only on spelling (i.e., not thinking about pronunciation at all).
The following are all correct:
a once-in-a-lifetime opportunity (“once” sounds like it begins with “w”)
an opportunity
a unique umbrella (“unique” sounds like it begins with “y”)