Some more info on this:

a) The bug will only happen if you create a new morph to a figure that already had a morph when you loaded the scene. If its a new scene, a new figure, or a figure with no prior morphs in a loaded scene, the bug won't happen.

b) The bug will never happen if you morph different body parts. But if your new morph touches a body part that has been previously morphed, the bug will happen.

c) You also can tell if the bug happened when you try to either load a new scene, start a new scene (File/New), or close Poser. Poser will crash if the bug happened and you tried to do either one of those.

d) The bug happens differently if you use the PMD file (Edit/General Preferences/Misc/Use external binary morph targets). Poser won't crash, and you won't get polygons flying all over your scene, but you'll get incorrect morphs.