For HN'ers who are still learning the finer points of English this title shows one of the many weird inconsistencies of the language.
Normally, when trying to say 'a thing' where the thing starts with a vowel we change the 'a' into 'an' so 'a axe' becomes 'an axe', 'a umbrella' becomes 'an umbrella'.
However this is a notable exception: when pronouncing 'user' there is an unwritten 'y' e.g. it is said 'yoozer', thus we wouldn't normally add another consonant here so it should be 'a user' not 'an user'. If you say it out loud you can hear it (and hopefully feel the extra tongue work that, traditionally, we try to avoid).
Another common exception is 'Hotel'. It takes 'an' rather than the expected 'a' as for some reason we drop the 'H'. For me it rolls better off the tongue with 'an' but this may purely be due to conditioning.
'Herb' is a difficult one as in the US it's pronounced 'erb' while in the UK it has a hard 'H'. Thus in the UK we use 'A herb' — I assume the US says 'an Herb' — correct me if I'm wrong.
It's nonsense like this that makes me pity you brave fellows who attempt to master our pidgin of a language!
Edit: change 'pigeon' to 'pidgin' :)