This page uses an older (from March 06) official version of jQuery. It works as you would expect.

The alert should only show one class set on the block at a time. Click the block to change its class.

The javascript gets its current class attribute with currentClass=block.getAttribute("class"); then does $(block).removeClass(currentClass).addClass(nextClass); to change classes.

There are five classes to cycle through, but with the latest version of jQuery the current class is not removed and the next class is added along side the un-removed first class, then successive clicks on the block fail to remove OR add any further classes.

Go to the identical test page using the latest version of jQuery