Today I looked to add to ATW. I entered a trade with the proper count, twelve shares, this was a conditional order, timing. Then i realized that the date was wrong and I cancelled the order and entered it again with the right date, but the default number of shares, 100. So the order went thru with 100 shares at $45.68, 20120827 10:16:19. I realized the error just 1.5 hours later and reversed it, selling 88 shares at 2012-08-27, 11:42:02, and $45.56 realizing a capitol loss of 88*.12 = $10.56 and 1 dollar commission, for a total loss on the mistake of $11.56 or .00001% of the 850K portfolio.
Disclosure: I am long ATW.
Additional disclosure: Less than 1% of the portfolio value.