The API for adding a custom psuedo selector which accepts an argument changed in jQuery 1.8. Prior to 1.8, one could do the following in order to create an :icontains(text) pseudo selector:

jQuery.expr[':'].icontains = function( obj, index, meta, stack ) {

	return (
		obj.textContent ||
		obj.innerText ||
		jQuery(obj).text() ||
		''
	).toLowerCase().indexOf(meta[3].toLowerCase()) >= 0;

};

In jQuery 1.8 this triggers a TypeError as the function parameters have effectively changed and the meta parameter is now undefined.

The API for creating a custom psuedo selector which accepts an argument is completely different in 1.8 and is not back-compatible with the method used for 1.7.

In my opinion, a change like this which is not back-compatible with the previous method is classed as a bug. A plugin which adds a pseudo-selector like this now must be updated for jQuery 1.8, and if it wishes to retain support for jQuery 1.7 will have to introduce logic to determine which method to use to add the pseudo selector.