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On the PC front you're going to find a lot of motherboards have a single 16 chanel connector and then a bunch of single channel. Perhaps one 4 channel. At least the motherboards I've seen however, use short connectors for the lesser channels. Apple's new systems, even though they offer 16,4,8,4, have full-length connectors for each slot. This enables you to stick a standard PCIe graphics card into any slot and run it at the maximum speed for that slot (the 16x board will just run slower in a slower slot).

I've seen pictures of a wacky PC motherboard which whilst having short slots didn't actually close off the ends on some of them so you could plug a longer card in with part of the edge connector exposed. Looked really reliable and secure...