The question of SP manipulation comes up here frequently, and i'm still just beginning to observe of the interesting and questionable trading patterns which are often discussed here in terms of SGR trading. So i think this excerpt on the topic of "spoof trading orders" from Chuck Butler column at Kitco.com today might be relevant or at least interesting or educational for some of us:

Two large banks (I won’t mention their names) were sued by an investor claiming they manipulated silver futures and options prices in violation of U.S. antitrust law by placing “spoof” trading orders.

For those of you who have never heard of “spoof” trading orders… these so-called spoof trading orders, are the submission of a large order which is not executed but influences prices and is then withdrawn before it reasonably can be executed.”