[Tom Line]

These answers are going to vary largely depending on your robot, however, we have a pretty solid system modeled after how we set up spares for Industrial machines.

The only protection you can have against failure is preparation.

End up having spares of all fasteners, bushing, bearings, and other COTS.
Have multiple spares of any item that you deem is likely to break.
Have single spares of any item that might break, but probably won't.
Have single spares of any item that is very hard to make.
Finally, evaluate your robot for any critical components. You absolutely MUST have spares for these, and if any are likely to break, you should have multiple spares.

Evaluate the tools you will need in your pit to fix anything on your robot - including a major structural failure. Spare aluminum tubing in multiple sizes is an absolute must for quick bolt-on fixes and reinforcement.

Have your pit crew train ahead of time - have them sit for an hour with the robot and discuss how to fix and replace every item on it in the best and quickest manner.