Repeat of bug ID: 1999126. This bug was marked closed but is still in 2.7.1. A simple subtraction of two rectangular blocks make a non-manifold shape.
Create a flat box 100 x 100 x 5 at (0,0,0)
then subtract a taller box 20 x 100 x 20 at (50, - 10, 0)
Exporting as STL fails :-
validate: number of faces (30) is not 2/3 the number of edges (48)
validate: calculated holes (using Euler's) = 1
Fixing in the solid editor makes it worse.