I was working for HP doing warranty repair/troubleshooting. We get a PC with no video. We tried replacing just about every component in the box and were scratching our heads to isolate what component it was. I suspected since we were doing one component at a time, that it might actually be two bad components and suggested both the motherboard and video card might both be bad. I explained that in assembly, they often didn't properly secure the video card in the AGP slot, and the during testing, they would plug the PC into monitors to run diagnostics. Pluggin in, and pulling out cables to an unsecured card while it is plugged in can cause the card to lift up, and then reseat while power is running through it.
My coworker said, "oh really?" and pulled the card out, and reseated it while the PC was plugged in. She then said, "I just finished troubleshooting the problem. Now I'm convinced it is a bad video card and motherboard."