Performing slice on an array of arrays results in unexpected behavior. The returned slice is altering the elements.

Examples:
> db.slicetest.insert(

{items: [["a", 1], ["b", 2], ["c", 3]]}

)
> db.slicetest.find()

{ "_id" : ObjectId("4be05dbad9e167ecfebc17ce"), "items" : [ [ "a", 1 ], [ "b", 2 ], [ "c", 3 ] ] }

> db.slicetest.find({}, {items: {$slice: 1}})

{ "_id" : ObjectId("4be05dbad9e167ecfebc17ce"), "items" : [ [ "a" ] ] }

This one is correct:
> db.slicetest.find({}, {items: {$slice: 2}})

{ "_id" : ObjectId("4be05dbad9e167ecfebc17ce"), "items" : [ [ "a", 1 ], [ "b", 2 ] ] }

> db.slicetest.find({}, {items: {$slice: [1,1]}})

{ "_id" : ObjectId("4be05dbad9e167ecfebc17ce"), "items" : [ [ 2 ] ] }

> db.slicetest.find({}, {items: {$slice: [2,1]}})

{ "_id" : ObjectId("4be05dbad9e167ecfebc17ce"), "items" : [ [ ] ] }

> db.slicetest.find({}, {items: {$slice: [1,2]}})

{ "_id" : ObjectId("4be05dbad9e167ecfebc17ce"), "items" : [ [ 2 ], [ 3 ] ] }

Issue reported on mongo-user:
http://groups.google.com/group/mongodb-user/browse_thread/thread/e84758d6a77ead8b