Jed McCaleb built a beta release of a Magic trading card exchange for the MtGox domain. He then read about bitcoin in a Slashdot article posted on July 11th, 2010 after which he decided to write an exchange. McCaleb insists that the bitcoin exchange was completely different from the Magic cards exchange, but Mt Gox went live as a Bitcoin exchange July 18th, 2010.

So either McCaleb built a brand new exchange from the ground up in one week, or he reused code from his Magic card trading service.

McCaleb sold the site to Karpeles 8 months later, and 3 months after that, it was breached for the first time. Allegedly, the hacker used McCaleb's old admin credentials to arbitrarily assign himself any amount of bitcoin, which he then started selling off to crash the price. Since the price crashed to $0.01, the dollar value of the withdrawal limit represented several thousand bitcoin, which the attacker promptly sent off-site.

No matter if the site was reused code from a Magic card exchange, or was written from the ground up, it never should have been within a thousand miles of anything of value.