How do you buy 10,000,000,000 (let alone 10x that much) of something as a junior trader without someone signing off on it? There's nothing at all to stop catastrophic actions being taken by anyone in the organization?
At the time, no. There wasn't any checking. And while it looks like a lot, it wasn't insanely expensive to unwind. Sometimes these kind of mistakes make money. My own 'mistake profit /loss' was around +1m USD.
Yen was probably around hundredth of euro or dollar. So it is 100 million or billion worth of very liquid currency. Very likely among the most liquid trade pairs. As such you need to trade big to make money there. Get timing right and you make it, get it wrong and you lose some.