module object() {
     difference() {
          translate ([0, 0, 12]) cube(30, center=true);
          translate ([0, 0, 12]) cube ([20, 10, 10], center=true);
          translate ([0, 0, 3])  cylinder ( h=10, r1=8, r2=8, center=true);
     }

     difference() {
          translate ([0, 0, 0])  cube ([18,18,25], center=true);
          translate ([0, 0, 12]) cube ([20, 10, 10], center=true);
     }
}

minkowski() {
     object(); 
     cube(2);
}

CGAL error: assertion violation!
Expression : e_below != SHalfedge_handle()
File       : /Users/kintel/code/OpenSCAD/openscad/../libraries/install/include/CGAL/Nef_3/SNC_FM_decorator.h
Line       : 657
Explanation: 
Refer to the bug-reporting instructions at http://www.cgal.org/bug_report.html
OpenSCAD(42174,0x7fff73440310) malloc: *** error for object 0x7fd610d3bc10: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug