Inappropriate use of the letter N in phrases like 'an historic'.
The H is not silent and so using 'an' instead of 'a' makes it sound totally disjointed and is an uncomfortable mouthful to say.
Yes, I know it's how it's supposed to be written/said in standard English but in this case its simply wrong.
I think it originates from the 1800s when one didn't pronounce the 'h' in some words such as hotel. It was the posh way to not pronounce 'h'. Language changes, but really nowadays nobody uses, or at least not very many people use 'an' before an 'h'. Now, as you say, because many folk do pronounce the 'h', using 'an' is obsolete.
I used to get annoyed at 'definately' instead of 'definitely'. Now though I realise it's part of the changes that occur in language pronunciation and emphasis. It's hard to put extra emphasis on 'it', if you don't put it on 'def'. All great fun.