I ran into this today: depending on the tolerance settings (0.001 or 0.01) the intersection curve between the plane and the curved surface is planar or non-planar, respectively.
Interestingly, the incorrect non-planar result (obtained at 0.01 tolerance) consists of three segments. The middle segment is correct and the same as the one obtained at 0.001 tolerance, whereas the two outer segments seem to force the intersection curve to the edges of the curved surface in a non-planar fashion. Why does this happen? Is there a setting to prevent this?
I find this behavior a bit troubling, it looks like the developer went to great lengths to add curve segments that are not needed. But maybe there is another explanation?