Queries can return different results depending on whether an index exists or not, when Indexing on lists.

It is best illustrated with code:

> db.test.insert(

{a:[1,1,1]}

)
> db.test.insert(

{a:[1,1,2]}

)
> db.test.insert(

{a:[1,1,3]}

)
> db.test.insert(

{a:[1,2]}

)
> db.test.insert(

{a:[1,2,1]}

)
> db.test.insert(

{a:[2,1]}

)
> db.test.insert(

{a:[3]}

)

> db.test.find(

{'a.0':1}

)

{ "_id" : ObjectId("4d60ceef276d207f24f8c3b2"), "a" : [ 1, 1, 1 ] } { "_id" : ObjectId("4d60cef1276d207f24f8c3b3"), "a" : [ 1, 1, 2 ] } { "_id" : ObjectId("4d60cef2276d207f24f8c3b4"), "a" : [ 1, 1, 3 ] } { "_id" : ObjectId("4d60cef9276d207f24f8c3b5"), "a" : [ 1, 2 ] } { "_id" : ObjectId("4d60cf04276d207f24f8c3b6"), "a" : [ 1, 2, 1 ] }

Unless you add an index:
> db.test.ensureIndex(

{'a.0':1}

)
> db.test.find(

{'a.0':1}

)

Same query now returns nothing. Returning nothing is probably what is expected in this case.