Why dummies and lags are problems in growth theory, addendum

During my polemic last week against dummies and lags in growth theory, I covered some of the problems, but neglected to mention that their presence makes the resulting model unstable to future changes in circumstances. So if one of the determinants hidden in the dummy or the lagged term changes, then the model starts behaving in an unexpected way. It is the same problem as confronted economics in the 1970s with the apparent breakdown in the Phillips curve relationship and revision of the underlying methodology for deriving such relations.