Everyone has those little things that irritate them.  It could be something as simple as someone chewing their gum too loudly.  Regardless, we all have them and we're stuck with them.

I used to work retail and in the banking industry.  There is absolutely nothing that irritates me more than when someone is in line and comes up to the register/teller line and is gabbing away on their phone.  I'm not a mind reader, how am I supposed to know what you need?

On top of that, it's just plain rude and to this day it still bugs me.

What is your biggest pet peeve?