That was caused by an actual physical short (in space, conductive bits, usually soldier balls, floating around and shorting things was a big problem), which happened somewhat commonly. I think Apollo 12 nearly aborted the launch because the display was showing all 8s briefly, also caused by a short.

However there truly was never an unknown software bug discovered in flight. Lots of known software bugs had workarounds, partially because there was a hard deadline of about 4 months where the software couldn't be changed before the launch. Pretty amazing.