There appears to be an unphysical feature in the background prediction for opposite sign muons in the CMUP and CMX triggered sample.  Below is the plot of the delta R distribution between pairs of opposite sign soft muons in the 1 fb-1 CMUP triggered sample.

The strange feature is the spike in the background prediction below dR=0.1 for the light/light background.

We investigate this further by plotting the shapes of the background components separately.

The spike at very low dR is visible in the W+jets and Drell Yan MC samples.  One interesting feature to note is that the error bars on the bin with the spike are very large.  This indicates that the bin contains a small number of events with a very large weight which suggests there may be a problem with the scale factor for a some kind of rare event configuration.  Now I have to track down which events these are and figure out why the scale factor is so large.