Not sure if this is common knowledge or not, but it took me far too long to figure out. So posting in case it helps others.

Old trek dual sport with 8 speed Shimano Acera would skip one gear near the middle of the range, and be very noisy on those cogs.

I assumed I had a bent hanger… nope. Tried numerous other things to figure out what I had done to cause this.

Turned out to be the shift cable was attached on the wrong side of the cable clamp post, causing too sharp an angle for the cable and ever so slightly changing the pull ratio.

The only way I figured it out was that I remembered some obscure web page that talked about doing this on purpose as a hack to get incompatible shifter and derailleur to work together.

HTH someone.