It is bound to happen - you are going to take something apart and in the process of putting it back together you are either going to lose a part or have parts leftover with no clue as to what you did wrong in the assembly. Depending on the circumstances it might be necessary to get creative. Reader Thomas submitted the following text and picture regarding a hack for a missing spacer.
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So I've had to use this hack twice. The first time was on a race weekend. A fellow cyclist had worked on his bike the night before the race (a big no-no), changing one rear wheel for another. In the process of swapping his cassette from one freehub-body to another, he missed transferring the small spacer that goes on the spoke side of the cassette... leaving the cassette wobbling back and forth 3mm... giving him some serious shifting issues.
Turns out, those things are hard to fabricate find when you need them! My wife ended up suggested using an elastic hairband in lieu of that spacer, and it worked perfectly! Since then, I've used a rubber band on my own free-hub as a spacer. I don't think it's a permanent solution, but perfect in a pinch. It is hard to see, but look at the end of the arrow.