When animating an element that has a -webkit-box-shadow, artifact trails are left behind.
Created attachment 17006 [details] test case
You don't need to animate to see this. Shadows don't respect transforms, but the bounding box computations are unaware of this. Arguably, transforming a box should transform its shadow, but I can also imagine arguments to the contrary.
See also bug 22159, on focus ring smears.
Ha, this works fine now. Probably fixed via bug 19623
Actually I think it was fixed by making shadows respect transforms to the extent that they can in <http://trac.webkit.org/changeset/34377>.
...which was bug 19397.