Here we have three lines, with one segment attached to each line.
The the third segment, the one on the right, has a direction opposite the other two.
The blue rectangle is a correct loft, in which the parameter-0 points on the two segments
can be connected by a line that does not cross the parameter-1 points.
The two green triangles are produced by the lofting of the second and third segments.
Since the segments' direction do not match, you get a bow-tie pattern instead of a rectangle.
Please look at the source code of this page for more information.