Impossible to grab mask nodes when mask has specific shape
The mask becomes editable only when the mouse pointer falls into the external dashed border of the mask, but sometimes when the mask assumes some shapes the dashed border falls into the mask so that some nodes are not editable any more because they fall outside the area which activate the editing process. When this happens by mistake it's impossible to edit the nodes and fix the error so that the full mask has to be deleted.
See image: the two nodes on the right of the solid line are impossible to grab because they fall outside the dashed line.
(Maybe there is a way to recover this situation, I don't know, but still it's not intuitive)