You’re taught that whether to use “a” or “an” depends on whether it’s followed by a vowel or not. But remember: that’s determined by the spoken sound, not the alphabetic letter.
Sounds easy, but it’s evidently easy to forget. In particular for initialisms that are spoken out as letter names rather than as if they formed a word. Some examples from real-life correction jobs:
- an X-ray
- a one-stop shop (following sound is w)
- a US citizen (following sound is y)
- an hourly rate (a silent h)
- an LPG-powered vehicle
- an MMR injection
- an ytterbium-doped fibre (following sound is a short i)
- an HTML document
The obvious simple way to get this correct is to say it out loud to yourself.
Prevalence: low. But does occur regularly, with acronyms in particular. I’ve included it because a discussion on a Facebook group showed that it wasn’t clear to everyone.
Frequency: medium. Repeat instances in the same document are commonplace.
Native: no. A native speaker probably doesn’t overthink the problem.
Advanced Dunglish 
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