Well, they figured out who crashed the stock market for 36 minutes back in 2010. Guess what, it was a random dude trading from his house in London.
Order spoofing, where people make it look like they intend to sell stock, while totally not selling stocks, is the primary culprit. Why would someone want to do this you ask. Say I want to get a good price to buy stock. I can wait for it to go down, or I can make it look like everyone is selling. If it looks like everyone is selling, everyone goes " Well I should sell now so I don't have to sell later at a lower price". They then sell me their shares, I take my pretend sell orders off, and the market goes back to normal.
Can we really be assured that the global financial system is sound when one individual trading on a retail account can crash the market?