Test which produces these discontinuities are; 

var anim = new Animation(
    document.getElementById("b"), {left: ["100px", "500px"]},
    {iterationCount: 2, iterationDuration: 1.0});
var parGroup = new ParGroup([anim],
    {direction: "alternate", iterationDuration: 1.0, iterationCount: Infinity});

At t=1.0 (and t=2.0, 3.0, etc), the value of left will end up at 100px rather than the preferred 500px.