  Lensing of the CMB by modes that are larger than the size of the survey
dilates intrinsic scales in the temperature and polarization fields and
coherently shifts their observed power spectra with respect to the ensemble or
all-sky mean. The effect can be simply encapsulated as a contribution to the
power spectrum covariance matrix in accordance with the lensing trispectrum or
as an additional parameter, the mean convergence in the field, for parameter
estimation. It should be included for upcoming surveys that precisely measure
acoustic polarization features deep into the damping tail at multipoles of
$\ell \gtrsim 1500$ with less than $10\%$ of sky. Its omission may lead to
seemingly conflicting values for the angular scale of the sound horizon which
may then provide erroneous cosmological parameters when compared to baryon
acoustic oscillation measurements.
