There are many different reasons why people spend time in a coma: illness, injury, surgery, or a medically-induced coma for various reasons. Their memories of that time vary even more widely. Some don't recall anything. Some were not even aware of the passage of time. Some had very vivid and weird dreams that stayed with them for years. And some had experiences that strangely melded dreams with reality.
"I remember hearing a woman telling me to squeeze her hand. I found out later that they brought me out of the coma to see if I was brain dead. I squeezed her hand, and they put me back under for several more weeks. I remember being taken to a spaceship orbiting Earth so I could be repaired. I found out that I was actually a cyborg. While I was on the ship, something ruptured the hull, and the ship lost atmosphere and everything froze. I was frozen in place but fully aware that I was frozen and could not move. I was now certain that I would spend eternity fully awake and immobile on this ship, never able to shut down." —Keri1986
You'll also read strange cases of people whose personalities changed after a coma, and one patient who recovered to prove his brother wrong, in a list at Buzzfeed. There are even more such stories in the comments.
(Image credit: Mohsen Atayi)