So.... Unity 4.5 Physics Updates has done... ahem... interesting things. 4.3.4 = 2D collider attached to an object. Rotate object on X axis. Since collider is 2D, it does not rotate. It completely ignores rotation since it exists in a 2D world. 4.5 = 2D collider attached to object. Rotate object on X axis. Collider2D will 'shrink' to a relative size as if the object were actually rotating, albeit still 2D 'flat'. At 90 degrees X, collider is infinitely thin since you're looking at a 2D object 'plane' in a manner of speaking, and will actually not trigger collisions at all. Funny. Completely new behavior. Looks like all colliders will get their own gameobjects now. Also... it seems to have fixed the inconsistent behavior of previous Physics2D where if you interpolated collision on a rigidbody, you absolutely could not rotate the Z axis of a transform that had a Collider2D attached. So, perhaps before I go redoing a ton of stuff, I should ask - is this the new expected behavior, or is this an oversight? To Test: Create gameobject. Attach 2D collider. Rotate X. Watch it shrink in scene view. At 90X, it's effectively "gone". Doesn't even have to be running. Haven't created a bug because... I don't know if it is a bug.