When there are too many namespaces in a MongoDB deployment, the top command will return a document larger than the 16MB BSON limit.

You can reproduce this easily by starting a mongod (I highly recommend using mmapv1 as it has way less overhead per collection than wt does) and doing something like

  for(var j = 0; j < 200000; j++) {
   db.createCollection("collection__"+j);
  }

Running mongotop fails with the message:

mongotop --port 7777
2016-08-03T11:16:54.361-0500	Failed: BSONObj size: 40761145 (0x26DF739) is invalid. Size must be between 0 and 16793600(16MB) First element: note: "all times in microseconds"

The shell returns that same error when running the top command manually.