I've built a custom Arduino shield using a PCB and surface mount components. The board has a short between power (5V) and ground (GND) when it is connected to an Arduino Due.
When the shield is not connected, the short does not appear to be present. With a continuity checker on my multimeter and the board disconnected from the Arduino, I've tested for continuity between GND and all the shield pins (including 5V). I only get continuity where I am supposed to - on the GND pins. I also tested for continuity between the 5V pins and the other shield pins (including GND), and only get continuity where I am supposed to - on the 5V pins.
How can I debug this problem? Why doesn't the shield show a short to ground when it is not connected, but when it is connected, there is a short? What am I missing?