There are a few things you can do:
When it does that, do "edit undo" or "ctrl z" so it will undo the messed up anchor point without deselecting your mask path.
Sometimes you can change it back into a regular anchor point by clicking on it once (not sure if this always works).
Also, if you want to stop and then continue your mask without starting over you can. Deselect everything, you can do right click "deselect all" or just choose the 'normal edit tool' (the one on the left right above the pen tool) and click anywhere. Then select the last point by clicking on it, choose the pen tool and continue your mask. Make sure to select just the last point.
You can also use that method to deselect your path and continue the mask from a different anchor point, the other end for ex.
You could also adjust that point that's sticking out instead of deleting it. There is a purpose for those kinds of points, they can make curves instead of ridged straight lines.
Also DISABLE SNAPPING! You won't be able to adjust anything smoothly with snapping enabled in pan/crop.
Here are screen shots with steps explaining everything I just said.
