MLCS sells a simple fix for a problem I didn’t know existed — evidently some router guide bushings can vibrate loose under use. Their solution is to sell you a spring washer that keeps pressure on the bushing nut so it won’t turn during use.
I can’t speak from experience; I’ve barely used my guide bushings since I snagged them on sale for unbelievably cheap, but the problem seems plausible — otherwise, why would people buy any sort of lock washer? These washers seem to be targeted at the Porter Cable-style bushings; I verified this by measuring my almost-new set and found that indeed the 1-3/16″ washer would fit.
MLCS sells two of these spring washers for $6. You might be able to do better in a hardware catalog, but 1-3/16″ doesn’t strike me as an easy-to-find size.
Spring Washer [MLCS]