Stock market spoofers put in orders to buy with the intent to cancel. This can shift prices up with fake interest, or it can shift prices down with a wave of cancellations. The spoofers then take advantage of the shifts by buying and selling accordingly. Bloomberg has an interesting stepper that walks you through the process for how one might catch such spoofers.
It starts with an overview. A minute of buying, selling, and cancellations whiz across the screen, and all looks hunky-dory. But then it zooms in on the details to show you what to look for, and it doesn't look like such a flurry anymore.
The challenge is that regular people cancel orders all the time, and the activity itself is not illegal. More data needed.