Be a Pal and answer this. I just had a dream that ended with me asking a lady of my acquaintance this question. Suppose one planet, Earth for instance, were to crash into another. Wouldn't it bounce, perhaps in our case all the way to Venus? Shouldn't some small percentage of planets have done that at some point in history? Of course a much smaller planet, say Neptune, could crash into a planet like Saturn and merge with it, but wouldn't that leave a huge and very different type of crater than those left by asteroids, so that a good microscope could tell the difference? In fact, if a liquid planet (say molten magma) were to crash into a larger planet and get wedged in, shouldn't it look very different from the traditional oblate spheroid for a while?
