When you query for a value on each of two arrays, both with positional operators, the $ projection operator can get pretty confused but smoothly performs the wrong operation.

Steps to reproduce:

Perform the following operations in the shell:

db.foo.insert( { a : [5, 6, 7, 8], b : [ 11, 12, 13, 14] } )
db.foo.find( { a : 7, b : 11 }, { "a.$" : 1 , _id : 0 } 

Expected result:

or an exception

Actual result :

Related documentation:

The following doc tells me not to do exactly what I did: http://docs.mongodb.org/manual/reference/operator/projection/positional/#proj._S_

"Only one array field can appear in the query document; i.e. the following query is incorrect:" (then the example shows me exactly what I did), but this is clearly a case where either an exception should be thrown, or else the $ operator should be made smarter.