  Since the work of Ferrenberg et al.[PRL 69, (1992)] some pseudo random number
generators are known to yield wrong results in cluster Monte Carlo simulations.
In this contribution the fundamental mechanism behind this failure is
discussed. Almost all random number generators calculate a new pseudo random
number $x_i$ from preceding values, $x_i = f(x_{i-1}, x_{i-2},..., x_{i-q})$.
Failure of these generators in cluster Monte Carlo simulations and related
experiments can be attributed to the low entropy of the production rule $f()$
conditioned on the statistics of the input values $x_{i-1},...,x_{i-q}$. Being
a measure only of the arithmetic operations in the generator rule, the
conditional entropy is independent of the lag in the recurrence or the period
of the sequence. In that sense it measures a more profound quality of a random
number generator than empirical tests with their limited horizon.
