The server has many read-only commands which do not validate their collection names. Our documentation mentions that empty collection names, or collection names containing a "$" are not allowed, so commands like the following should probably error instead of returning no results:

db.runCommand({aggregate: "$foo", pipeline: []})
db["$a"].find() 
db["$a"].distinct("a")
db["$a"].count()

Original Description

http://docs.mongodb.org/manual/reference/limits/ mentions that collection names containing $ or being empty are not allowed. OTOH, aggregate takes them as argument without complaining. Aggregating over an invalid namespace should not be a legal operation, in my opinion.

Examples:

> var cursor = db.runCommand({aggregate: "$foo", pipeline: [], cursor : {batchSize: 50}})
> cursor
{
	"cursor" : {
		"id" : NumberLong(0),
		"ns" : "test.$foo",
		"firstBatch" : [ ]
	},
	"ok" : 1
}

> var cursor = db.runCommand({aggregate: "", pipeline: [], cursor : {batchSize: 50}})
> cursor
{
	"cursor" : {
		"id" : NumberLong(0),
		"ns" : "test.",
		"firstBatch" : [ ]
	},
	"ok" : 1
}