Last year I entered a good to cancel order to buy Enlay.pk at some ridiculous low price like $4.25, which had dropped below that during the meltdown. At the time the stock was in the mid or high 5s where it has stayed.
But one day whomever the specialist is dropped down and picked up my order, selling to me for $4.25. But then of course I got a notice that the trade had been cancelled.
Of course, because the trade error WAS IN MY FAVOR, NOT THEIRS.
Make an offer to buy a $5 stock for $7 - by mistake - and see it picked up. And then ask for a cancel because your ERROR COST YOU MONEY..
Disclosure: no current position enlay.pk