This image illustrates the problem using trapezoids on top of a large box. Again, it's a cross section view. The trapezoid on the left will break off easily because it completely runs into the Simplify3D shell bug on the right and left walls. The trapezoid on the right will be much stronger because it avoids the Simplify3D shell bug on both it's right and left walls.
Complex models can have this shell bug all over them if they have similar vertical angles present in them, producing very weak prints.
I HIGHLY ADVISE THE SIMPLIFY3D TEAM TO LOOK AT THIS SHELL STITCHING BUG! It is causing a lot of the weak print complaints people are talking about. This is definitely a weak point in the software (pun intended).
These images illustrate the shell to shell bonding problem for boxes on top of a large box. They are cross section views showing the internals, with infill in the first image and no infill in the second image to help illustrate the problem. Then non-slanted box on the top right only has shell to shell bonds for 1 shell. This is the weakest piece. It will be very easy to break it off, despite having 50% infill. The boxes to the left have slanted walls that increase as we get further to the left. As the slanted box walls get closer to forming a horizontal line with the surface below it, the shells get closer to having completed bonds, and so the right walls of the boxes get harder to break off. As the slanted walls get further from forming a horizontal line, the shells begin having less bonds, and it will go all the way down to only 1 shell bonding once it's perpendicular, so in all cases above, the left walls are easy to break. A 1 shell bond is extremely easy to break.