Over on the CFZ blog, Neil Arnold has an interesting post on some sort of ophidian nightmare. The story's pretty neat, and not altogether unlike the experiences that take place in a hypnagogic or hypnapompic state with the crushing sensations. Sure, there are differences (for instance, Neil reports the ability to move) and I find it very interesting that while Neil's girlfriend didn't see the apparition, she experienced the same phenomenon.
In the past, I've done a lot of thinking on these hallucinatory states and how they may account for a lot of the stranger creature sightings out there. For those who aren't familiar with the terms, they refer to a type of hallucination that is sometimes experienced either when falling asleep (hypnagogic) or immediately upon waking (hypnapompic). These nearly always occur in conjunction with sleep paralysis, where a person's mind and body don't "sync up" in their states of wakefulness which results in an inability of the muscles to move although the surroundings can be perceived normally. This state of neither sleep nor wakefulness results in imagery from dreams being seen in normal surroundings. Through means not necessarily explained, the hallucinations can occur in conjunction with sensory phenomena, such as a feeling of being pricked by needles. Obviously, some researchers have connected this phenomena with many of the ghost sightings and UFO abduction stories out there.
I myself have had experiences at least twice with similar states. Once, in about 1995, I awoke quite late at night to find myself unable to move, with a black silhouette of a man, his arms held rigidly at his sides, standing in a corner of my bedroom. After a few moments, he vanished. On another occasion, as I lay waking up, I distinctly felt someone sit on my bed and place their arms on my shoulders.
This phenomena, I can vouch, is indeed very disturbing and frightening.
These states aren't fully understood. Personally, I would not at all doubt there could be variants of the phenomena where the individual is fully mobile. Likewise, given the physical sensations sometimes associated with the hallucinations, I wouldn't rule out the possibility that through some psychosomatic mechanism the marks associated could manifest on the body.
Perhaps these hallucinatory states don't even necessarily occur with sleep, perhaps it's possible to have them during waking hours, as well. To bring this to a more Fortean context, perhaps through some unknown mechanism the hallucinations can manifest physically and lead others to have experiences with an image ripped from someone else's dream.