Weird shell bug

 From:  jbshorty
Be careful when using "shell". Moi doesn't warn you, but it often will create invalid objects (OK for illustrations, but NOT OK for manufacturing or serious ind design). Invalid objects will make it impossible to do some volumetric operations later. Usually the problem occurs when a fillet is a 3-sided surface. It has much less problems if you take care that all sub-surfaces are 4-sided...