You insert a document with two array fields: db.test.insert( { "importing" : [ { "foo" : "a" }, { "foo" : "b" } ], "jobs" : [ { "status" : "completed" } ] } ) You try to query it, where you use $slice on one, and $ on another. The query should match the second element in importing, which does not exist in jobs: db.test.find( { "importing.foo" : "b" }, { jobs: { '$slice' : -1 }, 'importing.$' : 1 }) You get: error: { "$err" : "positional operator element mismatch" , "code" : 16353 }