I remember one time (post-wbtb?) where I must have done a relaxation exercise whilst on my back. I must have started to drift off into a hypanagogic-type state near to sleep. Thing is I must have had my mouth open a touch because the next thing I noticed my jaw must have (involuntarily) tightened as I suddenly felt and heard my teeth slam shut! (I don't think it was a hallucination either.)
It startled and worried me quite a bit at the time. I found it very disconcerting to say the least.

If you don't mind I think I will sit this one out.