Skewed Spreads
One practice (which, again, is thankfully less common now than it was a few years ago) is to skew the spread in the direction of the market move. In other words, if the market is trending rapidly down, most people are selling, so, although the spread may be the same, the prices quoted will be that much lower than in the real market, as most people would be selling into a down move. As soon as the market looks like it's going to recover, the skew is switched around the other way. This can shave several points off a client's profit. However, it can backfire in that not only does it upset customers by appearing unfair.
CMC is still doing this even now. During market hours spread on Ftse100 is 2 and after hours (Uk) is 3 and after 9pm spread goes to 6. But After Uk market closes, CMC in addition to increasing the spreads, skewing the spreads 4-6 more poinst in the direction it will suit their positions in the book. It happened yesterday and 2 times last week. CMC is clearly a bucket shop, uses every fraudulent trading tricks to make unfair profit.