Comment by crote

To me it sounds like they acted on the sale before it was finished.

Imagine you see Taylor Swift tickets for sale, so you start buying them. Meanwhile, you also start booking a hotel room. The Taylor Swift purchase fails because they are sold out by the time you try to finish your purchase - but at that point the hotel room booking has already succeeded! Should you be able to blame either Ms. Swift or the hotel for having to pay a cancellation fee on the now-worthless room?

From an automated trading perspective it makes sense to execute both orders at once as it minimizes the amount of time you have to hold onto it (during which your potential gain can evaporate or even turn into a loss), but it does expose you to the risk of cancellation. To me that sounds like the cost of doing business?