Comment 3 for bug 310206

I didn't really understand sketch mode when I filed this bug, but after reading "it might not be drawn closed the second time it is sketched", I tried it again. Interesting to note that if you draw the same path multiple times, all closed then it will crash, but if you draw even one of the paths open, it works fine, without a crash

I would say that if all the paths are drawn closed, then close the path, since it's obvious that if the user makes the effort to close each sketch then they want it closed.

Perhaps you could flesh out the explanation here for others who don't get it:
http://wiki.inkscape.org/wiki/index.php/ReleaseNotes047#Pencil_sketch_mode