I might be able to write a book on the things that I've "learned the hard way" over the years of doing this but a few that come to mind...
Pencils and chopsticks are great for removing an orifice that is stuck inside the inlet fitting of a 2nd stage. Not so great if the orifice is made of plastic and its REALLY stuck. That's a great way to crack or split a plastic orifice.
Inspect all critical threaded pieces and springs for cracks, preferably with magnification. Will you likely find a crack? No. Will it save a serious problem from occurring if you do find one? Yes. (I had a old MK20 that must have had a crack in the turret retaining screw as it sheared off a few days later when the kit was pulled out of the water by grabbing the 1st stage. Also found a handful of main springs from 1st stages with dangerous cracks in them)
Test everything in a leak bucket before you send it out, even if you didn't work on it. Not all leaks can be heard, but all leaks can be seen.
AquaLung Airsource 3's... do NOT overtighten the inflate button screw, not even a little. It will crack and fall apart in your hands.
Apeks Flight (and now the Ocea also) 2nd stages, hot water will help with getting the front cover off. tools are never the answer here.
MK25's IP adjustment screw, avoid running in through the cleaners. It will remove the gunk that helps hold the screw in place and when the piston applies upward force onto the seat, which in turn applies force to the adjustment screw, it will push the screw out raising the IP. This one is only really an issue after several times cleaning it but it will eventually be an issue.
ScubaPro springs with coatings (EVO & others). Never run them through an ultrasonic. They'll still work but they lose the coating and become more prone to rust & freezing.
Find yourself stumped on how to get something apart? Don't keep guessing. Stop, go read the manual, then come back. Strong chance that part is a reverse thread and you're about to break it better.
1st stage balance chambers are no joke. Learn the proper & safe technique to remove those O-rings or risk waiting for a new one to arrive. Additionally check these surfaces for scratches under magnification. If you find a scratch it will save you from wondering why your IP isn't stable after reassembly.
I'm sure there are more...