I discovered an unexpected KiCad issue after receiving new PCBs yesterday for an Arduino Shield project. After assembly, the board didn't work correctly.
Arduino Shield PCBs are easier to troubleshoot by modifying the code and commenting out features to disable them.
I traced the problem to a short between digital pin 12 and analog pin 1. It was not a solder bridge but a board-design error, which I confirmed by testing another unassembled board.
The short appeared only on the PCB, not in the schematic: the PCB contained a connection that the schematic did not. Updating the PCB from the schematic did not resolve it. The schematic's electrical rules check reported no errors, and the PCB's design rules check found no schematic parity errors.
An online search produced this explanation: “When the KiCad PCB editor shows a ratline (thin connection line) that does not exist or shouldn't be there, it usually means the schematic still defines a net connection that you removed or changed.”
The PCB contained an incorrect connection beginning at pin A1. To clear it, I removed the correctly displayed connection from the schematic, added a no-connect flag to A1, saved the schematic, and updated the PCB. After the bad PCB connection disappeared, I saved the PCB, restored the connection in the schematic, saved it, and updated the PCB again. The A1 connection is now correct. Once I finish reviewing the board, I will reorder the PCBs. This Arduino Shield design was developed in Tinkercad: https://www.tinkercad.com/things/57Vc8MxJXxy-tic-tac-toe-with-tri-color-leds
