"who the fuck hits someone with a ashtray"
Yeah, it’s supposed to be: “...with an ashtray.”
Gotta switch to “an” if the next word starts with “a”, I think. I believe it can’t be generalized for words starting with any vowel, like, “a one-time offer” but then when referring to another subject like “an officer”, it’s hit or miss. So, I think the hard rule is using “an” if the next word starts with the letter “A”. Any English majors out there to corroborate with this? (Joke: And I don’t mean officers in the British military)
More than a hard and fast grammatical rule, it's probably more practical to think of it in terms of how the words sound. If the following word sounds as if it starts with a vowel when you say it, then use "an". If it doesn't, use "a". Like in your example, "one" starts with a vowel when written, so it looks like it should use "an" on paper, but it's pronounced "won", so it uses "a". On the other hand, words like "honour" and "honest" both use "an", despite starting with consonants, because the H is silent. "Herb" is a funny one, because the British tend to pronounce the H, while Americans usually don't, so both "a herb" and "an herb" can be correct depending on dialect and accent.
There probably is a proper, more concrete rule out there, but as a general rule of thumb, I think you're usually safe to go by the pronounced sound over the written letter.