Enter a command for a keybinding. Make it a real long one and confusing one with may options and plenty of places for stupid typos and other mistakes or omissions. Well, after you've assigned the keyinding, it doesn't seem like you can edit it. Lokks like you've got to start from scratch. HOWEVER, you CAN use xconf-editor and edit the command there (not very intuitive though).
You can double click on the command in the treeview but I agree that this is not ideal, we should have an 'Edit' button.
Fixed in git master branch, commit 6b952f0a50dd17fb5d73b73bf37df9ac6519983c.