In The Prestige, when Angier visits Borden in prison, he asks him:
Don't they know you can't escape without your rubber ball?
(As can be seen in this clip.)
What does this dialogue mean?
He's mocking him in relation to the armed guards and cage and generally elaborate situation of his imprisonment.
The reveal of the film is that Christian Bale was actually portraying twin-brothers who created a magic act by pretending to publicly be a single man. He could "teleport" across the stage because one brother would go through one free-standing door on the left, and the other brother would walk out from behind the free-standing door on the right. To add to the illusion, the brother on the left would send a red bouncing ball across the stage for the other brother to catch it. It created a temporary distraction to make sure that the audience didn't look too closely at the doors, in case they noticed any motion. But also, the bouncing ball was touted in the act as being a magical item, and part of the overall magic of the "teleportation".
This act drove Hugh Jackman insane, because he believed it was the greatest magic trick in the world. He thought that Christian Bale was actually a single man either actually teleporting or doing some kind of advanced illusion, wherein both realities would be equally upsetting to Hugh Jackman. If he could teleport, then magic is real and Hugh Jackman has been doing nothing but parlor tricks in a universe he doesn't even begin to understand -- and this is what drove him towards connecting with Nikola Tesla (David Bowie). If it was just an illusion, then it would be an illusion so elegant and perfect and beyond anything Hugh Jackman could even comprehend, that his ego as a magician -- the greatest living entertainer -- would be shattered.
So, when Christian Bale is in prison and being treated like he's some kind of magical sorcerer who needs 'round-the-clock surveillance and extra security -- even though he's not even a very good magician by Jackman's standards -- then making a joke about how he "can't escape without [his] red ball" is just one more dig at him and his situation, insinuating that no one has anything to fear from him when he doesn't have his "magical" tools/trinkets.
It's a dig at him out of ego and depraved enjoyment of the circumstances.
It is a joke to the trick Borden use to perform with rubber ball.