Current state of spherepoly_check allows user to create polygons like spoly'{(0d,1d),(0d,2d),(0d,3d)}', which is incorrect because all points lie on the same meridian.
The root of this problem may be in sline_sline_pos fucntion, because it determines relations between sline'(0d,1d),(0d,2d)' and sline'(0d,3d),(0d,1d)' as "connects", which is also incorrect.

2 last log messages: pos=5 means lines are connected, but they actually overlap.

Solution I found is to change the order of block of code in sline_sline_pos responsible for detecting overlap and connection.
I created pull request with that fix.