That's not what I mean. The grammar rule isn't "a" before vowels and "an" before consonants, it is "a" before words that start with vowel sounds and "an" before

That's not what I mean. The grammar rule isn't "a" before vowels and "an" before consonants, it is "a" before words that start with vowel sounds and "an" before words that begin with consonant sounds, regardless of what letter the word actually begins with (e.g., "an hour" is correct, even though "hour" starts with an "H"). This is normal. What I'm talking about is when people reverse this rule to make themselves sound smarter. For example, I have worked with a few people during my career who would say things like, "I will need an pen and an paper to take notes." When I asked why they did that, they'd parrot back the grammar rule for who vs. whom, which is entirely wrong. This is what I mean by pretentious.