People who send out e-mall (not in response to any previous Subject) and put "Re:" at the beginning of the Subject line, even though the rest of the Subject line is brand spanking new. If it's a new stream of thought, replace the entire subject line! If it's somewhat related to the previous stream of thought, put the old subject in parentheses (after first prepending with "was:") and prepend the new subject, e.g., if the old subject was "My squeaky car", responses to that would be "Re: My squeaky car", deviations from that subject would be something like "cats in heat (was: Re: My squeaky car)" and a complete deviation, with no need to quote any previous post/e-mail might be something like, "fast cars, hot women".
(I realize I rarely change the Subject in a BEA thread. My explanation is since I can't actually split a portion of a thread into a new thread (ie, no moderator-like powers

) I find a change of subject to be distracting.)
People who don't know the difference between "e.g." and "i.e."
People who know how to use "Note:" but have never heard of "N.B.:".
People who put a (not insignificant) message into the Subject line of an e-mail, all while leaving the body of the message blank, or nearly so.
People who reply to your e-mail, but give no context. They don't mind paraphrasing (even if the references are vague or incorrect), but they hate the idea of actually quoting you, or using the more common "> ". (Gmail is a blessing here.) It could be days, weeks, or even months after you sent them an e-mail, and their response might simply be the cryptic, "Sure, that sounds good." Worse still is if they put "Re: your e-mail" in the Subject line. Bonus brain damage points.
People who send you an IM as though they were sending you an e-mail. You can see that they're typing (if the IM client supports that feature) but they don't stop typing till their client forbids them from entering any more. Then they hit Enter and continue.

People who are impatient in IM. They send you a message, and expect you to be completely transfixed on them. If you're multitasking or briefly interrupted by the phone or someone standing in front of you, their typical response is something like, "oh well, I guess you've gone. bye!" and suddenly they're offline. Had you been completely focused on them, they would have had no problem chatting your ear (eyes?) off for another half hour.
People who incorrectly refer to "\" as "slash" and "/" as "backslash".
People who verbalize "http://" as "aitch tee tee pee colon backslash backslash" (somewhat redundant, given the previous point).
People who write a URL beginning with "http:\\". Especially heinous when an organization (such as a TV news station) splashes this on the TV. Some chatter in BFE, I can frown upon, but when it's being broadcast to millions of folks across America/Canada/wherever, they're sending the message, "we know what we're doing, and this is absolutely correct".
Telephone support people who have far less technical skill than you, and insist on running you through their stoopid checklists before escalating you to someone who has a clue. "Have you rebooted Windows?" "I'm not running Windows, I'm running Linux. Er, I mean, yes, I've rebooted Windows."
People who have heinously poor spelling skills. Best example I've seen of late: "appsested" to mean, presumably, "obsessed". At least that's what I got from the context. On the plus side, such skills do have some entertainment value.
People who don't know when to use "role" vs. "roll".
Oh, so many gems, so little time. Need to get going here. Perhaps more after work.
