Sleep Paralysis might be one of the most horrifying experiences one could go through.
Personally i had it only once, now about a year ago i think; i really don't know what it was caused by, but i sure as hell am glad it has never occurred since. During my specific episode i was just unable to move - no hallucinations ensued - but it was still one of the worst things i've had the displeasure of feeling.
Now you might be wondering 'if this has happened to you long ago and never had it since, what prompted you to ask about it?'
Well, yesterday i ended up watching this video; now i'm morbidly fascinated by this horrifying yet somewhat captivating condition.