The Hardest Drive System To Program:

[basicxman]

Any base drive system is not incredibly difficult to program. Programming an effective control system for a drive system is hard.

see:

• Failure checking (state machine, redundancies)
• Human error correction
• Mechanical error correction

I really don't know what to answer your question with, everytime I think of a drive system I think of all the ways I could improve it with pieces of code - would be very difficult to pick a single one, and would require more context.