I’ve only just put up a new site (and just have two posts), and have noticed an issue with the presentation page slider. In both Chrome and Firefox, the slider will fade between slides 1 and 2, but when going back from slide 2 to 1, instead of fading, it will just blink back to the other image.
The built-in slider doesn’t play well with Jetpack’s Photon service because it fails to provide some of the necessary image file sizing information needed for calculations.
The issue is random and depends on how fast your browser/connection download the images from WordPress’ image caching servers.