  The idea of controlling the dynamics of spatially extended systems using a
small number of localized perturbations is very appealing - such a setup is
easy to implement in practice. However, when the distance between controllers
generating the perturbations becomes large, control fails due to increasing
sensitivity of the system to noise and nonlinearities. We show that this
failure is due to the fact that the evolution operator for the controlled
system becomes increasingly nonnormal as the distance between controllers
grows. This nonnormality is the result of control and can arise even for
systems whose evolution operator is normal in the absence of control.
