This one gets my goat. More so than people who say "pacific" when they mean "specific". And I've just heard it yet again on the news, where is seems even Sky's coffers won't stretch far enough to pay for someone who can speak properly.
"A" precedes words beginning with a consonant.
"An" precedes words beginning with a vowel.
The trouble with the letter H is, of course, that sometimes it is silent. So, what on paper should be "a honour" is actually pronounced "an 'onour". But that's easy enough to get your head round- if the H is silent, precede it with "an"; an hour, an honest person, an honourable act, an heir to the throne. If the H is pronounced, precede it with "a"; a hotel, a hot potato, a hierarchy, a history lesson. Easy.
It's pretty straightforward really. Yet somehow people- particularly news readers- just can't get their heads round it. Particular favourites are "an hospital" and "an historical event". Now if the newsreader was from Yorkshire, and dropped the H on everything, thus pronouncing "hospital" as " 'ospital" or "historical" as " 'istorical", then preceding with "an" would make sense. But they aren't and they don't and they just sound stupid. You can't have it both ways- pick one or the other but not both.
Maybe I'm unfairly directing the blame at the wrong person, and it should be the auto-cue programmer who deserves a few whacks with the cane before being made to stand in the corner. Either way someone can't speak, and given that much of the native English-speaking population struggle with the language as-is, the last thing they need is someone in a position of authority teaching them more bad habits.