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Just like that. Say you have a square and a circle, place them one above of other and convert both of them to paths. Do a union between those paths and you will see the round shape is not composed by two, three or four curve nodes but a lot of regular nodes. You don't get a rounded shape, you get a polygon with million of faces.

I did the same exercise between Karbon and Inkscape to show the difference: at the left is the result that Karbon returns (take a close look at the bottom, you will see that bunch of nodes I'm talking about) and the right is the result that Inkscape gives, where the rounded shape is built only by three curve nodes.

Moreover, sometimes both paths dissapear. I don't know why because I can't found a regular situation when that happens. But in my example, if you do an intersection you get the top portion of the circle built by only three nodes. In that case, that's the expected result.

I've talked about it before on KDE forums, they took me like crazy but I realize this is a serious problem. I don't know if it is really a problem that is related to QtSVG but I don't even know how I can report that bug.

Reproducible: Always

Steps to Reproduce:
1. Draw/paste two paths, one of them with curves.
2. Place one of them above other.
3. Do a boolean operation between them (Path->Exclude paths, Path->Intersect paths, Path->Substract paths, Path->Unite paths).
Actual Results:  
The curve part of the path is built by a bunch of regular nodes, that means it is actually a polygon with many faces, not a curve.

Expected Results:  
The curve part of the path should be made of a few curve nodes, the minimum amount of them required to draw the correct shape.