If a collection contains:

{}
{a: 1}
{a: []}
{a: [1]}
{a: [0, 1]}

... there are some bizarre semantics for how it's sorted. The empty array comes before the absence-of-a-field, and "1" comes between [0, 1] and [1]:

> db.collection.find({}, {_id: false}).sort({a: 1})
{ "a" : [ ] }
{  }
{ "a" : [  0,  1 ] }
{ "a" : 1 }
{ "a" : [  1 ] }

Reversing the sort does not quite reverse the order of results:

> db.collection.find({}, {_id: false}).sort({a: -1})
{ "a" : 1 }
{ "a" : [  1 ] }
{ "a" : [  0,  1 ] }
{  }
{ "a" : [ ] }

There are a number of tickets, some incomplete, regarding this confusing behavior:

• PYTHON-635
• SERVER-4346
• SERVER-5596