If you need to hold something to something else during fitting, masking tape can be useful. It comes off cleanly and extremely easily afterwards.
For example, the CPU cooler I use (Noctua NH-U12F) is screwed into a backplate. You put the screws into pressure springs and then screw them in - the springs ensure that it's impossible to over-tighten the screws. Since you must have the cooler above the motherboard until it's properly screwed in (obviously - it'll fall off otherwise), that means you have to put the screws in directly downwards. So the pressure springs will fall off the screws onto the motherboard unless you fasten them to the screws. A bit of masking tape does the job and can be easily and cleanly removed afterwards.
If you're bothered about using a magnetic screwdriver you can blu-tac screws to a screwdriver. I'd just use a magnetic screwdriver, though. They're not very magnetic. If you don't touch components with the screwdriver, you should be fine. I think that it's less risky than not using one because the risk of damage from a dropped screw is higher.
Off the top of my head, the biggest mistake I can remember making was spilling about half a pint of fizzy, sugary lemonade over a PC that was uncased and powered up because I'd just built it and was fiddling with settings. It was back in the days when things like voltages and clock speeds were set with jumpers on the motherboard, so I routinely left the case off until I was satisfied with the overclock. The PSU was untouched, fortunately, and the drives were only slightly splashed. The motherboard was awash, though, lemonade bubbling and dripping off everything. I disassembled it, washed everything in plain water, blow-dried it, left it overnight to be sure it was dry, reassembled it and powered it up. It worked fine.
Back in the jumpers day, I mistakenly put a 100% overclock on a CPU by mis-setting jumpers.
There used to be more ways to mess it up, with the heavyweight champion of the world being the motherboard power connectors on the AT power supply. Two seperate unkeyed identical connectors, plugged in side by side. Each one would connect on either side. Get them the wrong way around and your motherboard was ruined the instant power was supplied.
Eh, when I were a lad
