If anything, I would say "forgot" is (only slightly) more accusatory than "not notice", because "forgot" suggests a fault on his part, whereas, although "not notice" could be that too, it also allows the possibility that it was something about the way you presented it -- maybe it was in an appendix or a footnote or something that was easily overlooked. I agree with the others that your teacher seems to have a rather thin skin and that it should have been OK either way. However, if you know you need to be particularly sensitive with with this fellow, another way to soften such a remark is with extra words or to put as a question, e.g. "I think you may not have noticed ..." or "Perhaps you didn't notice ...?".