I boiled it down as much as I could. Test case is here: http://newlevant.com/marion/jqueryplugins/concentrationBug/ Calling animate like so: animate({opacity: 1.0}, flip.time, function () { ... }); And if flip.time is 0, subsequent calls don't wait for the animation to finish before calling the animate callback. Have not submitted a bug report (new to the whole system), but will if you think I should. (jQuery is fabulous by the way. Plugin where I found this bug is here: http://newlevant.com/marion/jqueryplugins/concentration/ ) Marion