Issue: On a Black Knight 2000, a display segment is always lit.
Solution: Tried the display board in another machine, which showed the same error, therefore isolating the error to the display board. Looked at the manual for the corresponding segment (see below - segment "m"). Probed the corresponding UDN7180A (U16 - 12) with a logic probe and saw the output signal was stuck for the affected pin. Probed the input pins of the UDN7180A, and they were pulsing correctly.
Replacing the affected UDN7180A solved the issue.