hard to explain, so look at the capx:
http://dl.dropbox.com/u/59009902/sine_bug.capx
for some reason sine size behavior doesnt take into account the frame aspect ratio, it takes the sprite aspect ratio.
what should happen is that the circle should remain circle and rectangle should remain rectangle. if you remove the sine behavior, aspect ratios remain ok after frame change through events.
hope my borked explanation helps :)
cheers