Spent 3 hours chasing a rear derailleur ghost in my own shop

Did you also spend a while staring at the cable and wondering if the housing was the problem? I did the same thing on a customer's bike last year, kept cranking the barrel adjuster and making it worse with every turn. It was a bent hanger too, and I felt like such an idiot because the tool was right there on the wall the whole time. Three hours of my life gone just because I skipped one check. Now the hanger gets checked first, no matter what the bike is doing. That lesson sticks with you forever once it burns you like that.