Are there exceptions as to when to use an before a word instead of a?

The article "an" is used before a word when the word begins with a vowel. "A" is used when the word begins with a consonant. The exceptions are when the word begins with a consonant but it sounds like a vowel, or when it begins with a vowel but it sounds like a consonant. There are very few exceptions. And I cannot think of one at the moment.