Found an issue with NumberLong when trying to work with the value 0. Inside a Map-Reduce function, I was trying to get a sum of values that included zero. Some of the resulting documents ended up with a value of null. After a long time of hunting, limited it down the fact that NumberLong(0) would kill a function if trying to convert to an integer (NumberLong.toNumber()) with no stack traces or log entries.

I would also note that I could find close to no documentation for NumberLong in the wiki.

Here's a test case that isolates the issue for me:

var test = function() { 
    print('NumberLong(0) test');
    var test1 = NumberLong(0);
    print(test1.toNumber());
    print('Unreached 1');
    // Also fails 
    print(test1 + 1);
    print('Unreached 2');
    return {'some' : 'result'};
};
test();

Expected (when run from the shell):

NumberLong(0) test
0
Unreached 1
1
Unreached 2
{ "some" : "result" }

Result:

NumberLong(0) test