Or idiot's mistake #6.. Look at it how you will :-)
As part of renovating my fleet of tired P4 wagons, a Coopercraft GWR open wagon with sheet rail came to hand. I've had a preliminary look at it, taken off the old S&W couplings that were not as well-fitted as the ones that I build now, and given the wheelsets a spin to see if there was any wobble.
There wasn't, but one wheel was catching very badly on the brakeshoe. Looking at the one on the other side, it was running well clear of the brakeshoe. Odd to find. So I popped the wheelset out, and looked at it carefully, and this is what I found.
Notice the difference? When I had checked, and presumably adjusted, the back-to-back setting on this wheelset I had made a simple mistake. I'd pushed the wheels along the axle until one was far back from the coning, and one was actually sitting over it. So although the wheelset was to the correct measured back-to-back, it didn't sit squarely in the chassis.
A lack of care on my part, and something to make sure that I don't do again in the future, but if you get poor running from a particular piece of rolling stock, it's something else that merits checking.