Changing crankshaft bearings with the engine in the car.
I've done it a few times when customers refused to pay to remove the engine. It's not difficult, but what is difficult is keeping it as clean as you need to. I always used a very small flat blade screwdriver to push the upper half of the bearing out. Once you get it pushed out about 1/2" you can just turn it the rest of the way out with your fingers.