once in grad school i was on a student committee who was finding TAs for a sequence of courses. for one of these courses, we had a guy in mind who we thought was the perfect fit for the job. we invited him to a planning session for the course sequence. after this session, after he saw what was being planned for the course we were thinking he would TA, he told me that he wasn't interested in TAing the course. so, we found other people (whom we also thought were great fits).

then, later, he became quite upset. he claimed he had never told me that he wasn't interested, and that i had promised him that he could be the TA, and that he had been counting upon it. i was completely surprised by his reaction. he didn't seem to believe that i thought he told me otherwise, and i think he felt quite betrayed by me.

well, of course, by that point the new people had been asked and agreed to do it, and were also counting on it; in my judgement he had been offered the position and turned it down; so we had to let them do it instead of him.

so, there must have been a miscommunication: he must have said something which he didn't think meant "i am no longer interested in this position", but which i thought meant exactly that. i didn't feel like there had been any ambiguity or uncertainty or interpretation at all, though.

the lesson i learned is: when an offer of consequence regarding a future plan is made, accepted, or refused, don't rely upon verbal communication alone, regardless of how clear you think the communication is. always follow up with a written email in order to confirm.