The SEC has enacted a new rule for busting (canceling) clearly erroneous stock trades. Previously each exchange has made its own rules.
The rules force exchanges to investigate potentially erroneous trades within 30 minutes, and to resolve the matter within 30 minutes thereafter.
As well, exchanges can only consider canceling a trade if the share price exceeds the last public sale price by more than 10 percent for shares priced under $25, by more than 5 percent for shares priced between $25 and $50, and by more than 3 percent for shares priced at more than $50.
See article here. Also, prior to this rule exchanges could take hours to cancel trades.
Disclosure: I have had about four trades busted by the Nasdaq, wiping out over $15,000 in profits. Two were on fat-fingered shorts, one was a buy of Google when the exchange was having computer issues, and one was a buy of GVBP in pre-market trading for half the bid price.