It just drives me crazy to see otherwise intelligent people use the word "a" where they should use an and some people ALWAYS use "a" and never use an. But that is not correct. Use 'an' if the next word starts with a vowel sound. If it does not, then use 'a'.
Examples:
Buy a house in an hour. (Although 'house' and 'hour' start with the same three letters (hou), one attracts 'a' and the other 'an'.)
An unknown goblin killed a unicorn.
An LRS... (LRS - Linear Recursive Sequence)
A TT race... (TT - Tourist Trophy)
It would be a honour.--this is incorrect ('honour' - starts with an o sound)
Send an US ambassador. incorrect ('US' - starts with a y sound)
She was involved in a RTA. incorrect ('RTA' - Road Traffic Accident)
Source: Grammar-Monster.com