  Lensing magnification and stacked shear measurements of galaxy clusters rely
on measuring the density of background galaxies behind the clusters. The most
common ways of measuring this quantity ignore the fact that some fraction of
the sky is obscured by the cluster galaxies themselves, reducing the area in
which background galaxies can be observed. We discuss the size of this effect
in the Sloan Digital Sky Survey (SDSS) and the Canada-France-Hawaii Telescope
Lensing Survey (CFHTLenS), finding a minimum 1 per cent effect at
$0.1h^{-1}$Mpc from the centers of clusters in SDSS; the effect is an order of
magnitude higher in CFHTLenS. The resulting biases on cluster mass and
concentration measurements are of the same order as the size of the obscuration
effect, which is below the statistical errors for cluster lensing in SDSS but
likely exceeds them for CFHTLenS. We also forecast the impact of this
systematic error on cluster mass and magnification measurements in several
upcoming surveys, and find that it typically exceeds the statistical errors. We
conclude that future surveys must account for this effect in stacked lensing
and magnification measurements in order to avoid being dominated by systematic
error.
