View: SP500 vs. Margin Debt As % of GDP

Forbes " During the dot-com bubble and housing bubble stock market cycles, margin debt peaked at roughly 2.75% of GDP. In the current stock market bubble, however, margin debt is nearly at 3% of GDP, which is quite concerning." http://www.finra.org/investors/margin-statistics shows 652.8 billion  
Debit Balances in Customers' Securities Margin Accounts. For comparison market cap of SPY is only 243 billion ... 
What could go wrong?