When running certain aggregation operations, it's possible to hit the following error:
2017-08-14T16:16:08.382-0400 F - [conn1] Invariant failure !index.sparse src/mongo/db/query/index_bounds_builder.cpp 306 2017-08-14T16:16:08.382-0400 F - [conn1] ***aborting after invariant() failure 2017-08-14T16:16:08.401-0400 F - [conn1] Got signal: 6 (Abort trap: 6).