Apply the attached patch (to limit the amount of memory available to the Sort stage) and then run the following:

    var largeStr = '';
    for (var i = 0; i < 32 * 1024; ++i) {
        largeStr += 'x';
    }
    for (var i = 0; i < 100; ++i) {
        assert.writeOK(coll.insert({a: largeStr, b: i}));
    }
    var res = db.explainsort.explain('executionStats').aggregate([{$sort: {a:1}}])

explain won't catch the "exceeded memory" error, and instead fail.

The entire explain will also fail if we throw an exception from the inner executor here.

To fix this, we may want to add a try-catch around the code where explain actually executes the plan, and add a new field like "executionSuccess" to the top level of explain's output.