Jeff and I have turned some wrenches in the past. Here's what's gone wrong.
2004 Dodge Stratus - Front Passenger's side wheel bearing. Got it apart, took it to shop to get old bearing pressed out and new bearing pressed in. Took it back to Jeff's, reassembled. Jeff says "Man, it was way too easy to get that thing back in." Then we looked around and noticed that the axle was still laying under the car.
Took it back apart, put the axle back in, and reassembled again...in about 10 minutes.
2000 Ford Ranger - Replacing rear axle assembly. The differential locked up due to an oil leak. I bought a new axle assembly from a junk yard. We removed the drive shaft, jacked the truck up, removed the wheels, removed the shocks, removed the nuts from the U bolts on the leaf springs, removed the torsion rod, removed the brake lines. Needed about an inch more clearance to get the axle out from under the truck, so cranked the jack one more crank.
We had forgotten to chock the front wheels, so the truck rolled forward off the jack stands and fell to the ground in a massive heap with the rear bumper on the floor of the garage and our two good jacks under it. Luckily neither of us was under the truck at the time, and we had another jack we could use to get the truck off the floor.
So, yeah...asking "what could go wrong" is probably not going to inspire a lot of confidence.