  We give an example of a vector bundle E on a relative curve C --> Spec Z such
that the restriction to the generic fiber in characteristic zero is semistable
but such that the restriction to positive characteristic p is not strongly
semistable for infinitely many prime numbers. Moreover, under the hypothesis
that there exist infinitely many Sophie Germain primes, there are also examples
such that the density of primes with non strongly semistable reduction is
arbitrarily high.
