I'm not reading or posting much these days but I have been pointed to this thread. Exchanges are regulators around the world are increasingly putting the onus on brokers to maintain what they deem to be an orderly market place. Of course in general when someone submits a market order you want to be filled at market. But what if the bid/ask if a stock trading at say 10 cents becomes thin and a submission through the market drives the price to 15 cents? In nominal terms that is nothing. In percentage terms, that's 50%. some exchanges would consider allowing that order to go through a breach and hit you with substantial fines. That's just one extreme example but I use that as a way to try to explain what brokers have to deal with. It really is no fun preparing responses on why a certain order was filled at a various price levels. Some exchanges do have volatility or other price limit controls but those tend to be at the wider end of the spectrum and do not limit a firms liability or prevent regulatory scrutiny. The short of it, these controls are put in place for a host of reasons including the need to limit fat finger and other disruptive practices, regulatory requirements and also a desire to reduce regulatory issues/costs and finally bad fills. You might be complaining about the odd order you don't get filled but what you are not hearing about are the numerous out of range trades that the filters prevented.