I get irritated when someone just says, "Oh, Bardiac will do something" as if that binds me. For example (and this is made up), if a colleague told a student, "don't worry, you can overload into Bardiac's class. No problem." without actually consulting me, and then expected me to overload the student so that they wouldn't "look bad." There's always this part about not making the other person look bad by letting them impose.
Last semester, one of my colleagues (A) told a committee that we needed to do something a specific way (X) because another colleague (B) had told a third colleague (C) that we would. But, we told A, we have no intention of doing X, and in fact X is counter to what makes sense. A said, but you have say that you are doing X or B will look bad. In other words, A wanted us to basically mislead C further to keep B from looking bad. For a moment, we dithered, and then we actualy stood our ground and said basically that B would either have to look bad, or actually tell C that s/he'd been mistaken about X. And that's what happened, and the world didn't end for either B or C.
And it's happened again (with different players). And I'm irritated.
I think there's a combination of active misleading, wishful thinking, and just plain not thinking going on, and an unwillingness to be responsible for making the occasional error. (We all make honest errors on occasion, and at least in English departments, these usually result in minor problems. That's one of the advantages of working on people who've been dead for 400 years. They never sue!)