A wide class of inflationary models, known as hybrid inflation models, may
produce topological defects during a phase transition at the end of the inf
lationary epoch. We point out that, if the energy scale of these defects is
close to that of grand unification, then their effect on cosmic structure
formation and the generation of microwave background anisotropies cannot be
ignored. Therefore, it is possible for structure to be seeded by a combina
tion of the adiabatic perturbations produced during inflation and active is
ocurvature perturbations produced by defects. Since the two mechanisms are
uncorrelated the power spectra can be computed by a weighted average of the
individual contributions. We investigate the possible observational conseq
uences of this with reference to general hybrid inflation models and also a
specific model based on supergravity. These mixed perturbation scenarios h
ave some novel observational consequences and these are discussed qualitati
vely.