"Like if you had the old pentium from years ago where sometimes the floating point processor returned the wrong answer?"
It goes back a bit further. Intel was having trouble getting the floating point side of the 486's to work consistently. Their fix was simple. If the floating point side of a chip didn't work when tested, they totally disabled the floating point side and renamed that particular chip a 486SX and were able to sell their scrap (slightly discounted of course). It was a brilliant solution. If the floating point side worked it was a 486DX.
Yes that's what I mean but the common point is not in the STL directly - there should be one line that the common point is on at the height of the current slice. Those "common lines" have a point at each end to define them. Those points are in two different triangles that should have a common edge (the common edge is the 3d line). It's hard to explain in words.
Anyway I don't think that's the problem. I think the calculation of all the line segments has some kind of floating point error. Like if you had the old pentium from years ago where sometimes the floating point processor returned the wrong answer? I'm not sure.