This does get confusing with say...

"A/An herb."

Because different dialects pronounce herb differently, sometimes the 'h' is pronounced, sometimes not.

I know you specified American English, but even within American English, you can find areas that differ on this, and I'm sure there are other words where this kind of thing crops up.

Also, I guess this may be worth mentioning as well:

Though this no longer seems to be as common as it was 10-20 years ago...

"An hero."

Sometimes, either an unintentional misuse of this a/an rule will be ironically copied, and more widely used, essentially to either mock the original usage/user...

... or the a/an rule can be intentionally misused, as a way of infantilizing yourself, trying to come off as cutesy/ditzy, or maybe play up your own awkardness or inexperience, something like that.