  The fundamental problem in the study of parallel-server systems is that of
finding and analyzing `good' routing policies of arriving jobs to the servers.
It is well known that, if full information regarding the workload process is
available to a central dispatcher, then the {\em join the shortest workload}
(JSW) policy, which assigns jobs to the server with the least workload, is the
optimal assignment policy, in that it maximizes server utilization, and thus
minimizes sojourn times. The {\em join the shortest queue} (JSQ) policy is an
efficient dispatching policy when information is available only on the number
of jobs with each of the servers, but not on their service requirements. If
information on the state of the system is not available, other dispatching
policies need to be employed, such as the power-of-$d$ routing policy, in which
each arriving job joins the shortest among $d \ge 1$ queues sampled uniformly
at random. (Under this latter policy, the system is known as {\em the
supermarket model}.) In this paper we study the stability question of parallel
server systems assuming that routing errors occur, so that arrivals may be
routed to the `wrong' (not to the smallest) queue with a positive probability.
We show that, even if a `non-idling' dispatching policy is employed, under
which new arrivals are always routed to an idle server, if any is available,
the performance of the system can be much worse than under the policy that
chooses one of the servers uniformly at random. More specifically, we prove
that the usual traffic intensity $\rho < 1$ does not guarantee that the system
is stable.
