  Weak gravitational lensing has become an important method to determine the
masses of galaxy clusters. The intrinsic shapes of the galaxies are a dominant
source of uncertainty, but there are other limitations to the precision that
can be achieved. In this paper we revisit a typically ignored source of
uncertainty: structure along the line-of sight. Using results from the
Millennium Simulation we confirm the validity of analytical calculations that
have shown that such random projections are particularly important for studies
of the cluster density profile. In general the contribution of large-scale
structure to the total error budget is comparable to the statistical errors. We
find that the precision of the mass measurement can be improved only slightly
by modelling the large-scale structure using readily available data.
