TLDR; English (like all languages) is crazy ... but one particular bit of craziness, the a/an split (and the exceptions to a simple understanding of it) isn't as crazy.
For those who won't read the article, either the word "a" or the word "an" is used before a word. Which is used depends on the first sound of the next word (not whether the next word starts with a vowel, as is commonly understood).
With this rule, only "129 of the 32,455 words in my list needed exceptions.", which is surprisingly regular.
I can't think of a single true example to the real rule, it's an actually extraordinarily strong and simple rule as English goes.