The number of errors found on the Yahoo! front page is not small. And most errors recur, like this misuse of the indefinite article an:
I don’t know why the writer chose to use an instead of a. Perhaps he or she thought that because the word following (which would be one) starts with a vowel, it must be preceded with an. That would be incorrect. The choice between a and an depends on the sound of the word that follows it, not the first letter of the word. If the word sounds like it begins with a consonant, then use a, not an.