Marilyn admits that she made a mistake
Marilyn is Wrong Copyright © 19951998 Herb Weiner. All rights reserved.
Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine,
published by PARADE, 711 Third Avenue, New York, NY 10017, USA.
According to Parade, Marilyn vos Savant is listed in the
"Guinness Book of World Records Hall of Fame" for "Highest IQ."
In her Parade Magazine column of December 24, 1989, Marilyn admitted
that she was mistaken when she reported that
"two automobiles crashing headon at 50 miles per hour each was equal in severity to one car crashing at 100 miles per hour into a stone wall."
Thank you, Marilyn
I'm pleased to know that you're willing to admit that you're wrong once you understand your error.
Now, if only I could convince you of your error in the other examples I've cited.
A stone wall is not an immovable object, and that
hitting a stone wall at 50 miles per hour will do some damage to the wall thus decreasing
the damage to the car that hits it.
By the way, if you let me build the wall, I'd be happy to run a car through it.
Further Explanations
GAC Philippe <phg@clubinternet.fr>
suggested some further clarifications:

Momentum is mass multiplied by velocity, and is a vector quantity (magnitude and direction).
The momentum is conserved but can be redistributed between the objects.
If two cars of equal mass are moving towards each other at equal speeds,
the total momentum is zero (because the vectors cancel each other out),
and the cars are stationary after the crash.
If the cars (or a car and a stone wall) do not have equal mass or speed,
the total momentum may not be zero, and the objects will not be stationary
after the crash.

Kinetic Energy is one half of mass multiplied by the square of velocity.
Although the total energy is conserved, it can be transformed from one form to another;
for example, from kinetic energy to heat, sound, or light.
In order for the objects to be stationary after the crash,
all of the kinetic energy must be converted to other forms.
Since the kinetic energy is proportional to the square of the velocity,
doubling the velocity quadruples the kinetic energy.
Therefore, one car traveling at 100 MPH has twice the kinetic energy
of two cars of equal mass, each traveling at 50 MPH.
http://www.wiskit.com/marilyn/mistake.html last updated June 30, 1998 by herbw@wiskit.com