If you are dealing with loose, nasty, dirty, and likely rusty upper control arms, and you are using a 3/4' drive breaker bar, this is a possible scenario:

a. UCA is in a vise. Ball joint won't budge.
b. Add a cheater pipe on the breaker bar. The ball joint still won't budge, but now the table that the vise is bolted to is moving.
c. Add a longer cheater pipe and anchor the table to the garage framing. Ball joint still won't move, but now the vise is bending the control arm.

Lesson:
Get an impact wrench on those ball joints to save damaging the control arms.

In my case, the UCAs were still on the car. But the forces needed to loosen the ball joints with a breaker bar were more than enough to push the car off the jack stands. An impact wrench was the solution.