It might be a personal problem, but the back part of my foot (kind of the Achilles tendon area/behind the ankle) was rubbing against the back collar of the shoe. It was really uncomfortable when I was driving and that's where I noticed it most, since pressing down on the gas pedal would push the back collar into the tendon area.
However after pushing through that and wearing it for the rest of the week, I think the material got softer, or that area of my foot/ankle got stronger, and I don't notice it anymore. The shoe is very well made, and I will probably buy another pair!