I just had it fail for me the other night -- or at least I think it did. This is the first time I remember doing an RC in a non-lucid dream, so I was pretty upset that it didn't work.

I was entering a bedroom devoid of furniture and my brother was standing there, so I thought to do the nose-pinch RC. As best as I can recall, I wasn't able to breathe, and I decided it wasn't a dream.

I'm wondering now if maybe a little bit of air leaked in through one nostril, and I just didn't pay attention because I know that's happened once or twice in real life.