The udpate7 test asserts that the documents with 'x' values 1,2,3, inserted in that order, return in the same order when sorted by _id.

function s() {
    return t.find().sort({_id: 1}).map(function(z) {
        return z.x;
    });
}
// snip
t.save({x: 1});
t.save({x: 2});
t.save({x: 3});
 
t.createIndex({x: 1});
 
assert.eq("1,2,3", s(), "F1");

This test fails if the _id increment happens to wrap around in the same second that all 3 documents are inserted.