Okay, I've got one I just remembered: what was it that made me so darn forgettable to one of my university lecturers?
This lecturer was our first year supervisor, so she dealt with our inductions and so on. We had a few months where we had less contact, then she took over one of the units, so we had her at least twice a week.
One day, I came into a lecture and sat down in the area I usually sat in, and this lecturer hurried over and asked me if I was the new transfer student she'd been expecting! And this wasn't at the start of a new term so she hadn't seen me in a while - I'd been in one of her lessons earlier that week! I informed her that actually I'd been in all her lectures to this point, and she looked slightly embarrassed and walked off.
About three months later, I was finding a seat for an exam when a teacher at the front announced that certain classes needed to be in certain number seats. I look and wouldn't you know it, not the right number seat. However, when I got up, I couldn't see any seats available in the correct numbers. I see the lecturer and go over to her and state my problem, and she smiled at me and went "Ok, what subject are you in?" I stared at her, stunned. "English. Yours." I replied. Not only had I never missed any of her twice a week lessons, there'd also been seminars once a fortnight and two revision sessions I'd attended for this very exam!
The thing that made it all the more frustrating was that it wasn't just a case of her being forgetful - she knew most of the other students by name, but for some reason she couldn't even remember that I was in her class!