The Master Lock doesn't live up to its name, according to Samy Kamkar, a well-known hacker who once developed a worm that brought MySpace down to its knees.
Now, he's exploiting a vulnerability in the supposed mighty Master Lock that becomes unlocked in fewer than eight or so tries. He discovered that after dissecting the lock's insides and seeing "the resistance he observed was caused by two lock parts that touched in a way that revealed important clues" about opening it, per Ars Technica.
In addition to that, he found that when the first and fourth digit in the combo is divided by four, it returns the same remainder. So, as demonstrated in the video, the lock's no-so-secret combination is cracked using a website he developed that spits out possible combination codes.
(The actual process of breaking it open is complicated, but it's clearly explained in the video.)
So yeah, Kamkar's discovery is worrying considering we place a lot of trust in these unbreakable devices. But judging by the video, the complicated process shows it likely won't be happening in your gym's locker room anytime soon. Unless you go to Kamkar's gym.