It looks like
DBQuery.prototype.comment clones a cursor and returns its first
result. And this seems different from other DBQuery functions that
call _addSpecial() just to modify an existing query (eg
DBQuery.prototype.sort)

DBQuery.prototype.comment = function (comment) {
    var n = this.clone();
    n._ensureSpecial();
    n._addSpecial("$comment", comment);
    return this.next();
}