Today I transplanted my system into a new case (White Define R4) and decided to use Bitfenix Alchemy sleeved extension cables (also white) to give my rig a white/black motif. I spent a fair bit of time with the cable management, and the case looked very neat. Posted fine on first boot.
Then the nightmare began.
System BSOD (haven't had one for 4 years...) and wouldn't post. I started troubleshooting by checking all connections and reseating everything (one by one and checking if it would boot), then started removing the Alchemy cables one by one. I removed the 8 Pin ATX, the very last of the Alchemy cables, and it booted as if nothing had been wrong with it.
It might only be the 8 pin, but I'm not about to spend another 2.5 hours finding out.
*Edit* I see I neglected to say they were extension cables in my OP. Sorry about that!
Only used them as extensions. It booted to desktop once and then blue screened. Rebooted and wouldn't even post. They can't even be used as normal cables.