This can be reproduced on the Tabs with Slideshow homepage on the jQuery tools page: http://jquerytools.org/documentation/tabs/slideshow.html

Press 'play' on the slideshow, watch a transition or two to see how long it should take to transition. Now click on 2 of the navigation icons quickly. Wait to watch the transitions. They double up and the intervals seems pretty strange. The more navigation icon clicks you perform before waiting for the slideshow to return, the more the intervals start to overlap.