Take this example of jQuery Core getting a invalid string in .show().

1. $("#ID").show("SlowAsFatCat");

Even with invalid string $.show() will still show the element with id of ID.

Example:  http://jsbin.com/acoxe4

If jQuery UI is loaded onto the page the $.show() method in the effects library will take over as it thinks the invalid string is a effect. But if it can't find the effect to show it does nothing. It doesn't fall back to doing anything or it doesn't display a JavaScript error.

Example:  http://jsbin.com/acoxe4/2

$.ui.show(); should provide the same consistent fallback as jQuery Core.