mongodump produces a metadata for a collection that contains undefined parameters, that stops mongorestore.
Exemple.
The collection message_queue :

> db.message_queue.stats()
{
"ns" : "nadb.message_queue",
"count" : 62,
"size" : 2840,
"avgObjSize" : 45.806451612903224,
"storageSize" : 4096,
"numExtents" : 1,
"nindexes" : 1,
"lastExtentSize" : 4096,
"paddingFactor" : 1,
"systemFlags" : 0,
"userFlags" : 0,
"totalIndexSize" : 8176,
"indexSizes" :

{ "time_1" : 8176 }

,
"capped" : true,
"max" : 2147483647,
"ok" : 1
}

Produces this message_queue.metadata.json :
{options :

{ "create" : "message_queue", "capped" : true, "size" : 1024, undefined }

, indexes:[{ "v" : 1, "key" :

{ "time" : 1 }

, "unique" : true, "ns" : "nadb.message_queue", "sparse" : true, "name" : "time_1" }]}