I do not have much experience with surface modelling, but I wanted to model a doubly curved shape and I thought this would be the easiest solution. I basically modeled my shape by making a surface loft between two curves and adding two guide curves. I then mirrored and knitted the resulting surface to get on symmetrical leave shape. I thickened it to make it into a solid.
The base shape looks like I want but when I now try to make an extruded cut through the body an unwanted surface appears extending from the sketch for the cut up to the body. Trying to make a cut with a more complex sketch gives the ever helpful 'failed due to a geometric condition' error. I think the weird result may have something to do with the body not being fully seen as a solid, or some of the surfaces not having knitted properly somewhere. I hope someone with more surface modelling experience can help me out with how to fix this.