People who either don't respond to e-mails, or take their sweet time doing so. I'm talking weeks, if not months, here. I once actually had someone reply to an e-mail I'd sent over a year before. Ugh! I want to kick these assholes right in the teeth!!
I know forwarding them the original e-mail with a raging, expletive-filled note would just make it worse, but I almost can't help myself. One of the supervisors at work (not my supervisor) has been sending my supervisor e-mails detailing things I have neglected to do (I've been really swamped), and my supervisor forwards them to me. I sent both of them kind of a tacky e-mail in response, which they both ignored, and even asked my supervisor to stop forwarding her e-mails because they're pissing me off, but she keeps doing it. Yesterday, I kept typing drafts of another tacky e-mail to send to the offending supervisor, and even contemplated getting a dozen eggs and some wet toilet paper and driving out to her house. I want to knock her in the goddamn head, I swear!