Example:
@con['tmp']['system.foo'].save({:a=>1})
=> BSON::ObjectId5088a0b7e382dd3b7800000a

The server is printing the following:
Wed Oct 24 19:16:04 [conn613] ERROR: attempt to insert in system namespace tmp.system.foo

I think the ruby driver should convey this, rather than returning an ObjectID. However, given that it checks the return status of each .command() call, perhaps this needs to be moved to be a server issue if the server is not propagating the error - I haven't investigated it deeply, just tried something obscure and got this unexpected behavior.