Forum users, Thank you for considering this. It appears to be an edge-case.
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6706 (Git)
Build type: Release
Python version: 2.7.8
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
but it also appears on other versions of FreeCAD which are available to me, including version 0.17
Submitted as Bug Report here: https://freecadweb.org/tracker/view.php?id=2904
1) Open a new document in Part Design
2) New sketch - a teardrop. This consists of
a) An arc of about 270 degrees centred at the origin
b) A line with a horizontal constraint
c) A line with a vertical constraint
d) Where one of the lines joins the end of the arc include a tangential constraint.
3) Pad the sketch.
4) On the new face draw a second identical sketch. Choose the curved part of the sketch as external geometry.
5) Pad the second sketch
At this point we should have a teardrop twice the height of the first, but the model falls apart. The results of the operation are inconsistent, but, put succinctly, the Teardrop Explodes.