1. "You couldn’t have suffered a brain injury if you didn’t lose consciousness." 

Not so.  A brain injury results from any traumatically induced disruption in brain function.  The victim needn’t have "blacked out" to have suffered a brain injury.  A disruption of brain function may be evidenced by the victim’s inability to recall events immediately before or after the accident, or any feelings of being dazed or confused after an accident.

2. "You couldn’t have suffered a brain injury if you didn’t hit your head."  

The brain can be injured anytime the head comes to a sudden stop.  The damage occurs when the brain moves inside the skull.  No impact to the head is needed.  If the head and neck makes a sufficient whipping motion, or if the head is shaken sufficiently hard, a brain injury can result..

3.  "Children recover from brain injuries better than adults do."

In fact, children often seem to do worse than adults. The reason is that, for a child, the injury is a "double hazard."  First, the child struggles in the first year or two to recover physically from the injury, if he can.  But even if his physical condition improves, he will find himself behind his peers in school.  Unable to catch up, he may simply fall further behind. 
If you don’t have any complaints, you’re fine.

4. "It’s just a concussion."  

Concussions, though considered "mild" brain injuries, are serious.  For most victims, their symptoms of a concussion will resolve within a year.  However, for a minority of victims — perhaps as many as 15% — their symptoms can persist for many years of for the rest of their lives, and can be devastating. 

5. "If you feel OK, then you’re fine."

Sometimes, the symptoms of a brain injury occur right away.  Sometimes, however, they can begin weeks later.  Frequently, a victim of a brain injury will suffer a serious loss — such as the loss of his sense of smell — and not even be aware of it until he is tested.