This entry in the manual: http://docs.mongodb.org/manual/reference/command/findAndModify/

at the end says "Warning When using findAndModify in a sharded environment, the query must contain the shard key for all operations against the shard cluster. findAndModify operations issued against mongos instances for non-sharded collections function normally."

We could specify that when using a hashed index simply specify the _id value, and/or we could modify this page: http://docs.mongodb.org/manual/tutorial/create-a-hashed-index/

Unsure if other pages may need to be updated.