Not wishing to disrupt the discussion that's ongoing in this thread, I think that what the OP meant was that sometimes "traumatic events" may cause you to completely forget - they may be so painful that as a defense mechanism your mind blocks any recall (the freudian repression) and they can only be recovered through theraphy or some drug like scopolamine or sodium penthatol. But other events may be so deeply ingrained in your mind that you keep recalling them vividly.
When we speak about stress we usually refer to a continuous situation that affects your emotional and or rational system. If light and if you have the resources to deal with it, it may highly positive, if it is felt as too overbearing and if you are in a fragile situation it may lead to a very disfunctional situation - for instance the expression burnout is applied to this situations - people loose their ability to control their emotions and become incapable of facing their day-to-day life.
Whatever happened with Mr and Mrs McCann and their friends, we don't know.
That's why I expected to see in Court some qualified expert giving testimony about the difficulties that the plaintiffs suffered.