You could write a story about someone who receives warnings of impending disaster in his sleep but can't remember them when he wakes up. The person/being delivering the warnings becomes more and more frustrated night by night and tries to get the dreamer to sleep-write messages, tie the sheets in knots, etcetera, but nothing works. Eventually the dreamer gets a tooth out under full anaesthetic, receives the message, and remembers it when (s)he comes round - but can't talk properly because of the extraction and fails to pass on the message to the authorities before the aliens invade/zombies rise from the grave/supervolcano erupts or whatever.