The Prestige/WMG

Lord Caldlow survived.

Or at least another Angier did. The two tricks that had much focus in the film, apart from the transporting man, were the catch-the-bullet trick and escaping from a tank of water. Considering that earlier in the film, Angier said "every night, I stood in that box, wondering if I would be the one on the stage, or under it", might it be possible that perhaps one night, even offstage or before the occurrences, he simply got... careless
Explanation of how this could work: Angier has the key on him when he's duplicated. The original thinks of it when he's in the box and gets out, then the blind stagehands move out the only slightly less heavy box. The prestige doesn't bother checking, either because it makes him feel guilty or he's so desensitized after twenty or so nights that he gets bored and doesn't bother checking, and intentionally (at least from his subconscious) brushes off thinking too much about the key.

The tank was locked from the outside.
- It's still possible that one night Angier forgot to lock it. It only had to happen once out of a hundred-show run.
- It's a trick tank; they show you the panel. The only difference is that a pick-guarded padlock is used instead of the show padlock.
Alternatively and relating to the below as well, Perhaps after killing the first clone, Angier realized that because of the risks, he should have a double, so he kept one, and just one, alive. And when Borden/Fallon killed Angier, that last Angier just... let it go, because he was done.
- Though if he was willing to trust a double, one wonders why they didn't simply perform the trick as Borden did using duplicates instead of twins.

Owens (Lord Caldlow's manservant) is Angier.

Look at how Owens looks. Look at how Angier looks. Now think back to the first replication scene where both Angiers go for the gun. Let's say that the original got the gun and got older, or even just used stage makeup. Angier might have anticipated Fallon's response to his twin's imprisonment and used an older looking clone to act as a failsafe in the event of any retaliation by Fallon.

Nicola Tesla is a Technocrat and a Magnificent Bastard

He planned the entire thing out to get rid of two pesky reality deviants.

Both the movie and novel take place in Assassin's Creed.

Tesla made the cloning machine based on what he learned from the POE he had for a while.

Borden or Fallon is a clone of the other

What if Borden wasn't lying when he told Angier that the secret was in Tesla? It's possible that Borden had gone to Tesla to get his help, but when he realized that Tesla's machine made a copy of himself, he understood that he didn't need to transport himself, he had the perfect doppelgänger, and could do a brilliant magic trick anyways.

Tesla also never denied making a machine for Borden, he could have made a machine and Borden could have found out how it worked without Tesla knowing, explaining why Tesla and his assistant didn't know exactly how the machine functioned.
It took me looking this page's history to make sure that I hadn't made this entry. I'm convinced this is true. It also adds new subtext to Borden's line:

He came in to demand an answer and I told him the truth. That I have fought with myself over that night, one half of me swearing blind that I tied a simple slipknot, the other half convinced that I tied the Langford double. I can never know for sure.

Borden's wife knew about the twin

Sarah told Olivia she needed to share something about Borden before killing herself, but Olivia never met with her to learn what it was. Sarah had deduced she was living with two men (which wouldn't be too hard, if she paid attention to small clues), and she wanted to let Olivia in on the secret before it tore her apart, too. Sarah hung herself not because her marraige had fallen apart, or because her husband had a mistress, but because she realized the man she loved had been playing an extremely cruel trick on her, with no end in sight.