Hi xrok1, I guess it's just the way it works...

If you have a curve and you extrude it, the extrude command creates a completely new object. You still have your original curve object, and now you have a new extruded shape as well.

It sounds like you are editing the curve and then having the extrude update through the history update function. The way this works is it is the same thing as if you deleted your old extrude and then ran the extrude command another time with your edited curve as the new input.

If you drag some points of your curve upwards so that your curve is non-planar, then extrude will not be able to put an end-cap at the top of your extrude. The same thing will happen if you try to do a brand new extrude on a non-planar curve as well.

That doesn't really have anything to do with objects being detached or not, it has to do with the extrude command itself only knowing how to place caps on planar openings. To cap a non-planar opening is much more difficult since there isn't just one shape that is easily defined to do it, you would need a surface that kind of bends and warps around, and it has to warp in a pretty accurate way to make sure that it touches the open edges so it can be joined there.

- Michael