Assuming the following data:

{
title : "this is my title" ,
author : "bob" ,
posted : new Date() ,
pageViews : 5 ,
tags : [ "fun" , "good" , "fun" ] ,
comments : [

{ author :"joe" , text : "this is cool" }

,

{ author :"sam" , text : "this is bad" }

],
other :

{ foo : 5 }

}

db.article.aggregate({ $project : { author : 1}}) ---- This runs fine

db.article.aggregate({ $project : { author : 1, _id : 0}}) ---- This also runs fine

db.article.aggregate({ $project : { author : 1, _id : 0}}, { $project :{"Writer" : "$author"}}) ---- This crashes the server.

After further investigation, it seems when I exclude the _id field in the last $project, things work as expected.

db.article.aggregate({ $project : { author : 1}}, { $project :{"Writer" : "$author", _id : 0}}) ---- This works as expected.