  When the Mersenne Twister made his first appearance in 1997 it was a powerful
example of how linear maps on $\mathbf F_2$ could be used to generate
pseudorandom numbers. In particular, the easiness with which generators with
long periods could be defined gave the Mersenne Twister a large following, in
spite of the fact that such long periods are not a measure of quality, and they
require a large amount of memory. Even at the time of its publication, several
defects of the Mersenne Twister were predictable, but they were somewhat
obscured by other interesting properties. Today the Mersenne Twister is the
default generator in C compilers, the Python language, the Maple mathematical
computation system, and in many other environments. Nonetheless, knowledge
accumulated in the last $20$ years suggests that the Mersenne Twister has, in
fact, severe defects, and should never be used as a general-purpose
pseudorandom number generator. Many of these results are folklore, or are
scattered through very specialized literature. This paper surveys these results
for the non-specialist, providing new, simple, understandable examples, and it
is intended as a guide for the final user, or for language implementors, so
that they can take an informed decision about whether to use the Mersenne
Twister or not.
