SPOILERS: Breaking glass
Just a random thought, but though Borden tying the Langford double knot was definitely a big contributor to Julia's death, so was that seemingly unbreakable glass! What good is it having an ax or sledgehammer there in case of emergency if the glass is so strong one or two blows doesn't shatter it?
We see the same again at the end when Borden tries to free Angier from the water tank. What gives? Not glass, apparently, even when slammed, with great force, by a large metal object.
They did not have safety or tempered glass back then, so the whole thing was a bit of a stretch on the "suspension of disbelief" scale. Especially happening twice.
No reason to have an ax/sledgehammer etc for emergencies if it doesn't actually do the job.