On a similar note, I've noticed a difference in use of a/an before abbreviations. I don't think there are any "official" rules on this, because it's never a problem orally.

Let's look at an example.

In my work, we talk about System-on-a-Chip, commonly abbreviated to SOC. Consider a textbook saying one of the following:

a) Blocks of a SOC are connected ...
b) Blocks of an SOC are connected ...

As far as I can see, both are perfectly legal. The difference is how the author would pronounce the abbreviation. SOC would be pronounced respectively:

a) sock
b) ess-oh-see