People who leave their phone numbers on my voicemail, but neglect to leave an area code. We don't all live in John Doe County, Wisconsin, ma'am.
Those who leave their complete phone numbers, but say them so fucking fast that I have to replay the message four times to get it written down.
People who don't know what their phone number is, and leave my voicemail on hold for 15 minutes while they're rooting around their office trying to figure their shit out.
Those whose letterhead stationery has their address with the state name written out completely. Yeah, your letterhead looks very classy, dipshit, but now I have to figure out what the postal abbreviation for your state is if I want to write you back. This goes double for those folks who live in one of the 87 states that start with M.
People who do not understand how to use the out-of-office auto reply when they're on vacation, and set it to "reply to all" instead of "reply to sender," thus ensuring that when an all-company e-mail goes out, we all get their damn auto reply. Then, all the other idiots who did the same thing send their auto reply to all the initial auto replies, and it goes back and forth until the fucking network goes down.
And in a non-work-themed annoyance:
People who think Washington, DC, is in Washington State. There are more of them than you would think. My sister-in-law is one...and she is a schoolteacher.
Updated to include:
My mother-in-law, when she says, "Don't go out in this chill without a sweater; you'll get sick." And all of the King's relatives and the King himself, who heartily agree with her that cold temperatures cause diseases. Have you heard of germs, people?