Possible error in CAN-Bus shield schematic?
No, it's not a mistake. It's related to this statement on their Wiki:


  When you use more than two CAN Bus Shield in one net, you should take the impedance into consideration. You should either cut P1 in the PCB with a knife, or just remove R3 on the PCB.


If you look closely at the pads of P1 you will see a small trace joining them together. That is the "rogue" green line you found. By cutting that trace you disconnect the termination resistor. You can then re-connect the termination resistor by soldering the pads together again.