jQuery is great until you make a typo in a selector. When developing, I want to be able to have console warnings for selectors that match no elements.

When you call $.debug() it could decorate an internal function that's used in the selection process. Pseudocode:

$.debug = function(){
    var original = $.someFunction;
    $.someFunction = function(selector){
        var ret = original.apply(this, arguments);
        if (ret == null) {
            console.warn(selector + " matches 0 elements";
        }
        return ret;
    }
}

(Sorry, I'm not very familiar with jQuery internals.)

A more complicated debug function would allow enabling/disabling it, so you could do something like:

$.debug(true);
$(".someclass").find(".somechild")
$.debug(false);

It could also optionally be set differently in the production build.

$.debug = $.noop

If this isn't likely to be implemented, I would appreciate the name of a good function to replace for this, so I can build my own development version.