I've had problems with "Power Cords" as well, it's usually the cord in my experience. I assume you tried several cords before the brute force insertion method? I received a faulty Power Cord for a small AC Adapter in an IDE <-> USB-2 Bridge kit recently and I've had others fail in the past.

ADB Cables with badly shaped or oversize surrounds can be problematic as well, sometimes it's the cable, other times it's poor PCB/Case alignment for recessed connector sockets.

Like the iMacs Dennis mentioned, some other Macs, especially the B&W 17" CRT Studio Display's recessed Power Cord socket, have had power cord/socket alignment problems. Switching cords a/o using the brute force insertion method have always been the answer for me.