Looking at db.serverStatus() it appears that opcounter values are overflowing and reporting as negative values after a particular threshold is reached. The query attribute below is an example of this behavior.

"opcounters" :

{ "insert" : 51352645, "query" : -779127550, "update" : 230051534, "delete" : 45755208, "getmore" : 214070596, "command" : 303271802 }

,