Another bug which seems to have started since updating OSX.
I used to create an Opacity mask. Clicking would add a point, and dragging would turn the next point into a curved masking line. They no longer curve like they used to. They're just harsh lines, and seems that you can go back over it all to make them curved, point by point, taking forever.
Also, hitting UNDO doesn't take away the last mask point, it gets rid of all of it.
ALSO, selecting a layer on the timeline, and then hitting the mask tool under Opacity, or whatever you want, it will create a shape layer instead. You click undo, and try again, making sure you've selected the layer you want, and it will drop a shape layer on the timeline instead.