Passing the string "return" to a JavaScript function apparently causes JavaScript to return abnormally. Here is the simplest example I have:

MongoDB shell version: 2.0.4
connecting to: test
> db.system.js.save({_id: "error20", value: function(a) {return a}})
> db.eval("error20(\"test\")")
test
> db.eval("error20(\"return\")")
null

Attached is a file showing the same function working for both node.js and the JavaScript interpreter in Chrome.