Unused prototype code and hardware that still exist

[KLOV]

SNESNESCUBE64's awesome work on Radar Scope's speech is what brought this topic to mind; What otherwise unused prototype hardware and/or code still exists on released boards?

Here's the ones I can think of off hand:

Missile Command - There's an unpopulated portion of the board that is there to drive lights on the prototype's marquee that responded to events in the game. Someone has reverse-engineered and gotten it to work though the code to drive it hasn't been found.

Donkey Kong -...

Read more

Source: https://forums.arcade-museum.com/thread ... st.532331/