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:
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.