Currently, makeArray() uses array.call to determine whether array is a function, which fails with scripts that define Array.prototype.call.

makeArray: function( array ) {
	var ret = [];

	if( array != null ){
		var i = array.length;
		//the window, strings and functions also have 'length'
		if( i == null || array.split || array.setInterval || array.call )
			ret[0] = array;
		else
			while( i )
				ret[--i] = array[i];
	}

	return ret;
},

Unfortunately, there are quite a few scripts do so, notably, effect.js (an old version)

Search for Array.prototype.call

Is it OK to use jQuery.isFunction(array) instead of a simple property test?

If jQuery.isFunction() introduces too much overhead, we can also use something like typeof array == 'function'.

Instead of testing whether array is not an Array, we can also test whether it is an Array.

e.g. if (array && array.length && array.splice && array.join)

This may seem unnecessary if the developer has full control of all javascript code but it makes jQuery more robust (good for bookmarklets and greasemonkey scripts).