The following projection operation should likely throw a user assertion (or return nothing) due to matching an embedded object instead of the intended array:

> db.a.save( { a:{b:1} } )
> db.a.find( { a:{b:1} }, { 'a.$':1 } )
{ "_id" : ObjectId("4fe0f094026e93e0aaef076d"), "a" : { } }