Mt. Gox was a bitcoin exchange based in Shibuya, Tokyo, Japan. Launched in July 2010, by 2013 and into 2014 it was handling over 70% of all bitcoin (BTC) transactions worldwide, as the largest bitcoin intermediary and the world's leading bitcoin exchange.
Mt. Gox announced that approximately 850,000 bitcoins belonging to customers and the company were missing and likely stolen, an amount valued at more than $450 million at the time
Today that amount of Bitcoin would be worth nearly $40 billion dollars (depending on the day of the week).
Reading the above makes me concerned. If the world's (once) largest exchange was unable to prevent a hack of this magnitude, how can any exchange (without a multi million dollar security budget) be reasonably certain that hack like this won't effect them?
Is there a consensus (or at least a plausible theory) as to what went wrong on the technical side that allowed such a large exchange to be hacked? What practical lessons can one learn from this story in terms of securing their users funds?