Answer by coffiarts

Meanwhile I found out that the bone rolls in my model are partially inconsistent: I did a test by automatically recalculating them (in Blender) and reimporting my model. As a result, my character looks weirdly twisted (as expected), but the animation issues are mostly gone (approximately 80% of the flapping effects have vanished). Hence, though it's still not completely solved, I strongly suspect my own corrupted Blender model being the reason for my problem. I assume that I have simply forgotten to recalculate my bone rolls in Blender prior to creating my animations. And now there's probably no easy way to revert to a proper state (other than recreating all my animations from scratch) :( **So the answer to my own question is: Take extreme care to properly clean up bone rolls in Blender before animating!** It seems that this is a potential reason for screwed up animation behaviour in Mecanim (i.e. it's most likely not a Mecanim problem, at least in my situation). How to recalculate bone roll in blender?: - switch to Object view, - select your armature - switch to Edit view - press "a" to select all - press space to bring up the context menu - type in "roll" to find the "Recalcutate bone roll" option Hope this helps!