That's likely what we're looking at here. The behavior of this particular program was picked up by market monitor Nanex, whose own algorithms mine trading data for anomalies. In this case, the program began placing orders early last week involving 500 stocks on the NASDAQ (the technology index, of course) in rapid-fire, 25-millisecond bursts--first 200 quotes, then 400, then more and more, repeating itself. But it canceled those orders just as quickly, never actually completing a trade. At 10:30 a.m. eastern time on Friday, the program vanished.