  We perform a cross validation of the cluster catalog selected by the
red-sequence Matched-filter Probabilistic Percolation algorithm (redMaPPer) in
Dark Energy Survey year 1 (DES-Y1) data by matching it with the
Sunyaev-Zel'dovich effect (SZE) selected cluster catalog from the South Pole
Telescope SPT-SZ survey. Of the 1005 redMaPPer selected clusters with measured
richness $\hat\lambda>40$ in the joint footprint, 207 are confirmed by SPT-SZ.
Using the mass information from the SZE signal, we calibrate the richness--mass
relation using a Bayesian cluster population model. We find a mass trend
$\lambda\propto M^{B}$ consistent with a linear relation ($B\sim1$), no
significant redshift evolution and an intrinsic scatter in richness of
$\sigma_{\lambda} = 0.22\pm0.06$. At low richness SPT-SZ confirms fewer
redMaPPer clusters than expected. We interpret this richness dependent deficit
in confirmed systems as due to the increased presence at low richness of low
mass objects not correctly accounted for by our richness-mass scatter model,
which we call contaminants. At a richness $\hat \lambda=40$, this population
makes up $>$12$\%$ (97.5 percentile) of the total population. Extrapolating
this to a measured richness $\hat \lambda=20$ yields $>$22$\%$ (97.5
percentile). With these contamination fractions, the predicted redMaPPer number
counts in different plausible cosmologies are compatible with the measured
abundance. The presence of such a population is also a plausible explanation
for the different mass trends ($B\sim0.75$) obtained from mass calibration
using purely optically selected clusters. The mean mass from stacked weak
lensing (WL) measurements suggests that these low mass contaminants are galaxy
groups with masses $\sim3$-$5\times 10^{13} $ M$_\odot$ which are beyond the
sensitivity of current SZE and X-ray surveys but a natural target for SPT-3G
and eROSITA.
