  Interacting theories with higher derivatives involve ghosts. They correspond
to instabilities that display themselves at the classical level. We notice that
comparatively "benign" mechanical higher-derivative systems exist where the
classical vacuum is stable with respect to small perturbations and the problems
appear only at the nonperturbative level. We argue that benign
higher-derivative field theories exist which are stable with respect to small
fluctuations with nonzero momenta. A particular example is the 6D N=2
higher-derivative SYM theory, which is finite and unitary at the perturbative
level. The inflation-like instability with respect to small fluctuations of
static modes is always present, however.
