Like a false warning in a coal mine causing a deadly stampede, the market's collective overreaction and rush for the exits is often the real source of damage, amplifying a minor shock into a major crisis. The panic itself is the poison.
The true catalyst for a global crisis isn't the size of the initial failing economy, like Greece. It's the resulting panic and lack of transparency in interconnected financial instruments like derivatives, which makes every major bank an 'unwitting cosigner' to the initial default.
When emerging economies borrow in U.S. dollars, they are unknowingly making a bet that oil prices will remain stable. A spike in oil strengthens the dollar and weakens their local currency, simultaneously making their debt more expensive to service just as energy import costs soar.
Financial crises are rarely caused by risks everyone is watching, like inflation (known knowns). The true danger comes from unforeseen events (unknown unknowns) like 9/11 or the Lehman collapse, which aren't priced into risk models and cause systemic panic.