Used to happen to me at work all the time. I'd show a co-worker how to perform a task and instead of remembering or even writing it down (or not using what I had written down for them) I'd get called over the next time the task came up. If I didn't do it, my reports to person would end up at my desk asking "why won't you help them, it's part of your job".
I don't have an anger problem, I have an idiot problem.