People who are brain dead make certain motions that make it look like they are aware of their surroundings. The most horrifying motion of all is the Lazarus sign — it can convince people that their loved one has literally come back from the dead.
The Lazarus sign is an example of a spinal reflex arc. Living people often experience spinal reflex arcs. If we accidentally press our hands against something sharp or something hot we will pull back before the sensation hits our brain. The sensory nerves send a signal to the spine which sends a signal to the motor nerves to get moving; it's only later that we feel what is was we drew back from. These reflexes don't involve the brain, and so they keep happening even when the brain is dead. Most of the reflexes are simple twitches, but the Lazarus sign is something else. It lasts several seconds — the arms rise up from the body, then draw in to the chest, sometimes crossing over the heart. After pulling in for a few seconds, the arms drop back to the body's sides. The Lazarus sign, some say, is why Egyptian sarcophagi show people with arms crossed over their chests.
No one knows exactly why the Lazarus sign is triggered, but it can happen in response to painful stimuli and it can happen in the last few moments before death. It most commonly happens after a brain-dead patient is taken off a respirator. This is one of the reasons it's called the Lazarus sign; it looks like a person is rising from the dead, and lasts so long that few lay people can convince themselves that it's an involuntary response. Obviously, it's incredibly upsetting for families who see a patient unhooked from a respirator seemingly struggle to life for a few moments, and then die. Some papers on the subject actually counsel hospital staff to take families out of the room just after the respirator is removed, so there's no confusion, false hope, or guilt. The body has a thousand strange and fascinating responses. Though this one, sadly, can cause a lot of pain to the survivors, it causes no pain to the person undergoing it.
Image: Everyone's Idle.