When running against a server with a large number of databases (in this case over 2,500), mongotop dies immediately:

$ mongotop
connected to: 127.0.0.1
assertion: 13106 nextSafe():

{ $err: "BSONObj size: 31507783 (0x1E0C547) is invalid. Size must be between 0 and 16793600(16MB)", code: 10334 }