Some things you never forget. I had a friend in a coma for a couple of months and saw him make his first movement; raising his hand and spreading open his fingers. Turns out he was trying to place a $5 bet as he could vaguely perceive the beeping and flashing lights in the hospital room and thought he was in Vegas. After no one took his bet he thought he must be at an LA Kings hockey game. Next day he spoke his first words: "I have cigarettes in my nose". His breathing tubes. Funny in hindsight but brain injuries are so unpredictable.
It can be understandably terrifying. Some, like my friend, are also reluctant to wake up as they are afraid they did something wrong. So the doctors advise that you regularly tell them where they are, they were in an accident, are unconscious and did nothing wrong.