replacing "failed" components question: causal or coincidental repairs?

I think repairing old computers is just like any human endeavor - it is flawed and sometimes I follow incorrect logic and/or misdiagnose before finding and fixing the real broken part. Sometimes good parts get mistakenly identified as bad. That's life and if a part falls under suspicion I am likely to just replace it just to test the theory, right or wrong it gives information and vital clues. Anything labeled "suspect" is pulled aside and labeled as bad regardless. These parts are cheap and plentiful enough that if a few good ones get trashed in the process of fixing an old machine I consider it a worthy investment. Probably I am tossing some good parts occasionally but it is worth it time wise to take the chance and maybe find the root cause. Were I more skilled technician maybe I could fault isolate with more precision and just fix exactly what is broken but this is just a hobby and I do not have the time/experience to get more precise.