Yes, but it's based on the word's pronunciation starting with a vowel sound, not the word's spelling starting with a vowel letter. In the word "user", the 'u' is normally pronounced as 'yu', which starts with a consonant; therefore the correct phrase is "a user", not "an user". Contrary to ReactiveJelly's speculation, this is true in both American and British dialects. Perhaps there is some other dialect where it isn't.
Comments
Yes, but it's based on the word's pronunciation starting with a vowel sound, not the word's spelling starting with a vowel letter. In the word "user", the 'u' is normally pronounced as 'yu', which starts with a consonant; therefore the correct phrase is "a user", not "an user". Contrary to ReactiveJelly's speculation, this is true in both American and British dialects. Perhaps there is some other dialect where it isn't.
Interestingly enough, also y is a vowel in many languages. This is not a surprising mistake to make if you are not a native speaker.