When an incorrect easing name/function is added as an option, it fails gracefully;

    $("#block").animate({
        width: "50%",
        easing: "schwing"  // incorrect easing name won't break the animation
    }, 1500);

But if it is added as a string, this and all subsequent animations fail to work:

    $("#block").animate({
        width: "100px"
    }, 1500, "schwing");  // incorrect easing name WILL break the animation

Demo here: http://jsfiddle.net/smSaS/

I believe this might fix it, but maybe someone who knows the core better can find a better solution:

Line 5853:

var defaultEasing = jQuery.isFunction($.easing[this.options.easing]) ? this.options.easing : (jQuery.easing.swing ? "swing" : "linear");