Bug#27 added to the known bugs list. If you use begin/endTransition and have a g:background element on your web page, the Gadget image will corrupt.
I've included a Repro for MS, a screenshot of what you see with a blank g:background and a workaround.

Bug #27 System.Gadget.begin/endTransition corrupts g:background
dlcollison
K, thanks. =)