In our production environment, we have a sharded cluster setup. Recently, querying for some data sets have started to fail due to this error:

Assertion failure holder.nextFreeOrSelf == i src/mongo/db/exec/working_set.cpp 68

This has not happened before, but now it is consistently happening for certain queries repeatedly. There's been no change to how we store or query the data. We are currently running 2.6.4. See attached log for more detail, but this is currently breaking a critical path in production.

Any insights you may be able to share on this would be much appreciated.