If evolution is constantly changing, how can a creature be caught off guard? The answer is that in a closed system, it is possible to reach a stable point that it is difficult to jump out of: in the search space of genotypes, there is a local maxima in the utility of the phenotype.
One thing I was wrong about was that the stable equilibria doesn't exist indefinitely; eventually a mutation might arise that does jump out. I was also assuming that a closed ecosystem was also closed to change in general. The latter seems more common; that equilibria is upset by an outside event that alters the utility of the phenotype - such as an open system closing, or some non-biological change in the environment (eg climate), and not by a change in the genotypes within the closed system.
It is possible that aliens are in one of these stable configurations at the time that we encounter them, which seems more likely in closed ecosystems. Closed ecosystems can be created by internal barriers being removed (which intelligent life, or at least we, seem to be doing). Of course, it's harder to eliminate change altogether (!), so that source of alteration in the utility function would remain. Yet, we certainly have motivation to eliminate environmental change that affects us adversely, so perhaps intelligent life is likely to be at equilibrium, in the long term.
If you disagree, I request that you indicate with which part. ;-)