Great advice, and clear writing style.

Story time! During my 2nd year of university, I built a small CPU as a project. It was split across 2 breadboards, using an FPGA for the control unit.

It worked fine during debugging, but sometimes it would give really weird problems. Then when we tried to debug again, it was fixed. What was going on?

We'd forgotten to connect a common GND between the breadboards. When we attached the probes to debug it, the ground was passed via the USB port on the PC.