I came across an unused routine today that's slightly more interesting. Firstly, OutRun stores the 'lap-time' of each stage you've completed - even though this information isn't really made use of (other than showing your previous lap-time on passing the checkpoint).
Either for debug purposes or as an unfinished feature the programmers included a routine to sequentially print these times during gameplay below the HUD.
Entering the following command in the mame debugger will print the lap-time for a particular stage you've already completed in a race.
d0 = x; pc = 7fea; g; [where x is the stage time to print]
Here's an example screen grab:
Here, I'm on Stage 3 and I've printed the lap-times for the previous two stages using the unused code routine. The lack of apostrophes in the additional lap print outs indicate that this idea was probably pulled at an early stage.
Code decompilation is progressing nicely. I'm on track to finish this year.