Sub penny bidding. https://www.bloomberg.com/view/articles/2015-01-15/ubss-dark-pool-was-shady https://www.bloomberg.com/view/articles/2015-01-15/ubss-dark...

Sub penny bidding. https://www.bloomberg.com/view/articles/2015-01-15/ubss-dark-pool-was-shady https://www.bloomberg.com/view/articles/2015-01-15/ubss-dark... Sorry if this isn't the best article on the topic, it was a quick google search. My friend got bit hard by this. He had a HFT algorithm that was essentially guaranteed to at least break even by providing liquidity in the market by doing some fancy tricks that he's used for embedded digital signal processing. Problem was, his bids weren't being fulfilled even though he'd made them hours, sometimes days, previously. Given the amount of information available to him it looked like his orders were first in line at a given strike price to have his orders filled once the price moved to him. The only thing he could figure out was that someone was able to submit subpenny orders that would fill before his. I was skeptical initially, then some weeks later it started making the news that dark pools were managing to make subpenny bids on various digital exchanges. Far as I know the SEC turned a blind eye since the dark pools already had special privelidge in exchange for being market makers and liquidity providers.