Given indexes:

{ a: 1, b: 1 }

and

{ a: 1, c: 1 }

, a range query on 'a' with a projection on

{ a: 1, c: 1, _id: 0 }

will choose the non-covering index

{ a: 1, b: 1 }

when it should choose the covering index

{ a: 1, c: 1 }

.

This happens because the optimizer doesn't consider the projection when selecting a plan, so it chooses the first plan that seems optimal (which is a: 1, b: 1 because that index appears first in the list of indexes for the namespace).

To reproduce:
db.a.ensureIndex(

{ a: 1, b: 1 }

)
db.a.ensureIndex(

{ a: 1, c: 1 }

)
db.a.insert(

{ a: 10, b: 10, c: 10 }

)
db.a.find({ a:

{ $gte: 10, $lte: 12 }

},

{ a: 1, c: 1, _id: 0 }

)

The current work-around is to add a hint for the proper covering index. It would be neat if the correct index was chosen without a hint, though.