Ran into a strange issue using jqueryUI highlight effect, the issue only occurred with Webkit browsers (Google Chrome and Safari) and only when I was manipulating the background property of the element that was being highlighted.
Using a slightly modified version of the demo. I tweak the background image before applying the highlight effect
$(this).css("background", "url(some_image.png) no-repeat");
Update 2009-03-06: You also need to make sure that if there is an existing style on the element being highlighted it is using the more verbose style tags of background-image and background-repeat or the same error will be encountered