Updated: Sep 26, 2020
Create a sphere, make it editable and enter Polygon mode. Select a few polygons and press ‘delete’. There is now a hole.
To fix it, this is what you do.
First, switch from Polygon mode to Point mode – and you’ll see that the points at all 4 corners of the polygons you deleted are still there. Weird.
Right click in the Viewport and choose ‘Polygon Pen’.
Then click on each of the four corners of one of the missing polygons, and a white replacement will appear.
Make sure you click on all four corners, because it will look complete after you click on three – but it’s not.
STOP PRESS: New in R20, Maxon decided that you could only see the 'old' polygon points if you switched them on. To do this, go to 'edit > preferences > modeling / sculpting' and change the 'Disconnected Points on Delete' setting to 'Preserve (press Shift to remove'). Then you'll see them.