Wiring of the LEDs for the colour display. This was done twice as the first time I forgot that the wires to the connections on an IDC cable for a 25-way D Type connector do not go to Pins 1, 2, 3 4, 5 etc., but to 1, 14, 2, 15, 3, etc.
So the long, fiddly job had to be completely redone - the second time I had it plugged in so that I could test each connection as it was made.