Quote from opt789:
...A market maker (or anyone else who sees it) can fill your spread order at any time and for any reason they want. I occasionally get really good fills on spreads, and other times I end up giving up because I canât get anyone to do it. When I was on the floor we would fill any order if it matched up on the screen of any other floor. So if the pit broker was holding a spread that we didnât want to do at the time, and another single order came in for one of those legs, the pit broker could try to fill one side of the spread with the single public order and the other leg with us.
I understand that the pit broker/market makers can fill the spread that way, but if the options are traded only on an electronic exchange (such as ISE) is there such possibility that the exchange's algorithm will recognize that a spread order limit fits to the aggregate limit of two individual legs and therefore fills the spread order?