  Hybrid functionals often improve considerably the accuracy of
density-functional calculations, in particular of quantities resulting from the
band structure. In plane-wave (PW) calculations this benefit comes at the cost
of an increase in computation time by several orders of magnitude. For this
reason, large-scale problems addressed within the PW formalism have to rely on
pre-relaxed atomistic geometries, obtained with cheaper local or semi-local
exchange-correlation functionals. We investigate how suitable these geometries
are when plugged into single-point hybrid-DFT calculations. Based on several
case studies, we find two important sources of error originating from (i) bond
strain and (ii) over-mixing between defect and crystalline states. The first
arises from the mismatch between the pre-relaxed geometry and that obtained
after a subsequent hybrid-DFT-level relaxation. The second occurs when defect
states edging an underestimated band gap artificially mix with crystalline
states, affecting the local bonding character of the defect, and therefore
leading the spurious hybrid-DFT energies. Due to cancelation effects, the
lingering strain contributes little ($\lesssim10$ meV) to the error bar of
quantities based on energy differences of pre-relaxed structures. The error
from state over-mixing does not benefit from cancelation effects and has to be
monitored with caution.
