The decision by exchange to cancel trades are absurd and outragous. Let's say if I have P & G stock and got stop out at 50 and market continued to go down and I bought back at 40 again. and later on PG recovered to 60. what happened to my trade? if they canceled my 40$ buy order, I lost. Market is market and you cannot just undo trades. who gave them power to undo the trades? if exchange can undo, why not individual investors undo trades?