'a/an: They are used before a singular noun that has a plural form. 'A' is used before a consonant sound and 'an' is used before a vowel sound.'
Despite the supposedly simple rule, determining the proper usage of 'a' and 'an', a lot of people, and more importantly many journalists, still fail to uphold the correct usage.
Examples that I have encountered recently are 'an spent smoke grenade' and 'an media', with many before then. The reason these two stick in my mind like flies to manure, is simply because 'spent' and 'media' do not have anything even resembling a 'vowel sound'.
The rule itself is of little difficulty to comprehend, unlike the offside rule in Football (or Soccer, to Americans), which makes it harder for me to accept the complete meltdown of the rule itself.
Words beginning in 'H' that originated in French are correctly paired with the indefinite article 'an' (e.g. an [h]otel). Yet words that have no such link are still incorrectly paired.
It is not a problem for me with casual chat, such as forum postings - rather when someone is getting paid to write, they should be able to write proficiently, and at least getting one of the simplest rules in the language correct.