I always pronounce the H in historic and no one taught me a rule in order to form a habit otherwise, so I say "a historic."
That being said, this really demonstrates the weakness of the H sound. It's there, but it almost isn't. Plenty of words have always dropped it (e.g., hour). Some words drop it for a large portion of the population (e.g., herbs). And, with some words, there's an urge to use "an" with it or drop it when using an indefinite pronoun. Plus, there's the stereotypical northern English accent that drops it always and Romantic languages (and I assume vulgar Latin) that have absolutely dropped it (I wonder why Spanish even has the letter anymore).
That being said, this really demonstrates the weakness of the H sound. It's there, but it almost isn't. Plenty of words have always dropped it (e.g., hour). Some words drop it for a large portion of the population (e.g., herbs). And, with some words, there's an urge to use "an" with it or drop it when using an indefinite pronoun. Plus, there's the stereotypical northern English accent that drops it always and Romantic languages (and I assume vulgar Latin) that have absolutely dropped it (I wonder why Spanish even has the letter anymore).