Grounding a Rack-Mounted Motherboard?

MadCow-ard asks: "Here is one for the Electrical Engineers among us: I have a rack-mount case that I had installed an Intel D845GBV and P4. I had to switch them out with an Asus A7V333 because of incompatibility with my other hardware (which is a long story for another day), but they were functioning fine otherwise. In doing so we found that the new motherboard would not work in the system. We checked everything: multiple motherboards, video, RAM, power supply, cables, you name it. We were getting AOK POST sometimes, others not. It would randomly boot and other times it had no video, or a partial boot. After going nuts (in the field with a couple of clients) we switched out the risers the motherboard sits on. Voila. The risers appear to have grounded the system board. Not the Intel, just the Asus. I spoke with another tech associate who claims to have seen the same issue recently. Now grounding I understand, but it seems that it wasn't the risers specifically. It was their height. We tested two, the bad ones were 2mm smaller. It could be the alloy, but I thought that motherboards would shield the screw points from grounding. It appears to have been a field that built between the case and motherboard due to the smaller risers. It wasn't actual motherboard contact with the case, I checked. Does this make sense? Has anyone else seen this? Is it some sort of capacitance with the case that is generated specifically from the board design and layout?"