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”)
- an umbrella
- an awful house
- a house
- a boring hour
- an hour (the “h” in “hour” is not pronounced)