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We’ve covered a lot of ground; make sure you haven’t forgotten anything. Fans that connect directly to the power supply and other remaining elements should be hooked up now. Bundle and tuck loose any unused cables. Check the computer for foreign objects.
The last one is only half joking; a single screw fallen into the wrong place can cause a short circuit. Leave your screwdriver or something in there and it could do a lot of damage rattling around when you move the case. The green rectangle in the picture shows where I left my phone after receiving an inconveniently timed call. Having forgotten that in the PC could have caused rattling damage and would have been creating all kinds of electro-magnetic interference. At least the problem would identify itself: Imagine hearing your muffled phone ringing and realizing you had to take your PC apart to answer it.
Make sure all of the wires are out of the way - you don’t want something to get pulled loose, or worse yet, broken; especially this late in the game.