  We present high signal-to-noise galaxy-galaxy lensing measurements of the
BOSS CMASS sample using 250 square degrees of weak lensing data from CFHTLenS
and CS82. We compare this signal with predictions from mock catalogs trained to
match observables including the stellar mass function and the projected and two
dimensional clustering of CMASS. We show that the clustering of CMASS, together
with standard models of the galaxy-halo connection, robustly predicts a lensing
signal that is 20-40% larger than observed. Detailed tests show that our
results are robust to a variety of systematic effects. Lowering the value of
$S_{\rm 8}=\sigma_{\rm 8} \sqrt{\Omega_{\rm m}/0.3}$ compared to Planck2015
reconciles the lensing with clustering. However, given the scale of our
measurement ($r<10$ $h^{-1}$ Mpc), other effects may also be at play and need
to be taken into consideration. We explore the impact of baryon physics,
assembly bias, massive neutrinos, and modifications to general relativity on
$\Delta\Sigma$ and show that several of these effects may be non-negligible
given the precision of our measurement. Disentangling cosmological effects from
the details of the galaxy-halo connection, the effects of baryons, and massive
neutrinos, is the next challenge facing joint lensing and clustering analyses.
This is especially true in the context of large galaxy samples from Baryon
Acoustic Oscillation surveys with precise measurements but complex selection
functions.
