It's really only happened once - my T had forgotten a really important thing that I'd told him, about the recent death of a family member, and so I had to tell him about it again. I couldn't believe he'd forget a thing like that, and I was rather disappointed about it. The next session I did bring it up; I hadn't been sure if I would be able to, but I'm really glad I did. I prefaced it by saying something like "After last session I was feeling a bit unhappy with the fact that you'd forgotten" - like miswimmy says, focusing on your feelings, and on the action you're disappointed about, rather than on the T's person. (Not that a T should not be able to deal with this kind of thing even if it isn't phrased perfectly.)