To properly interpret the phrase "when first base is occupied," append "at the time of the pitch" in your mind. It does not matter if the runner at first is stealing. For the purpose of this rule, she occupies first if that's where she was when the pitch was released. If there are two outs, status of a runner on first base doesn't matter, for the purpose of this rule.
Also keep in mind: if the pitch bounces and then goes cleanly into the catcher's mit or hand, that is still considered "uncaught," and the batter may attempt to reach first under the provisions of this rule.
The uncaught third strike rule puts a lot of pressure on teams -- especially catchers -- to know the rule, know the situation, and keep your head when everyone starts screaming bogus instructions to the batter, the catcher and the base runner.
One variation that even MLB players occasionally forget: Two outs, bases loaded on an uncaught third strike it's a force at home, so don't throw to first and risk an error that could score two or three runs.
