Spoofing: magically disappearing bid/ask walls (illegal)

Spoofing is a disruptive algorithmic trading activity employed by traders to outpace other market participants and to manipulate markets.[1][2][3][4] Spoofers feign interest in trading futures, stocks, and other products in financial markets creating an illusion of the demand and supply of the traded asset. In an order driven market,[jargon] spoofers post a relatively large number of limit orders on one side of the limit order book to make other market participants believe that there is pressure to sell (limit orders are posted on the offer side of the book) or to buy (limit orders are posted on the bid side of the book) the asset.
Spoofing may cause prices to change because the market interprets the one-sided pressure in the limit order book as a shift in the balance of the number of investors who wish to purchase or sell the asset, which causes prices to increase (more buyers than sellers) or prices to decline (more sellers than buyers).[5][6] Spoofers bid or offer with intent to cancel before the orders are filled. The flurry of activity around the buy or sell orders is intended to attract other traders to induce a particular market reaction. Spoofing can be a factor in the rise and fall of the price of shares and can be very profitable to the spoofer who can time buying and selling based on this manipulation.[2][7][8]
Under the 2010 Dodd–Frank Act, spoofing is defined as "the illegal practice of bidding or offering with intent to cancel before execution."[1][9] Spoofing can be used with layering algorithms and front-running,[10] activities which are also illegal.[1][3]
High-frequency trading, the primary form of algorithmic trading used in financial markets is very profitable as it deals in high volumes of transactions.[11][12][13][14][15][16] The five-year delay in arresting the lone spoofer, Navinder Singh Sarao, accused of exacerbating the 2010 Flash Crash—one of the most turbulent periods in the history of financial markets—has placed the self-regulatory bodies such as the Commodity Futures Trading Commission (CFTC) and Chicago Mercantile Exchange & Chicago Board of Trade under scrutiny. The CME was described as being in a "massively conflicted" position as they make huge profits from the HFT and algorithmic trading.[17]