Its nearly always the damn humans. I find it awful that we are often assigning blame to some "technology error" when its the damn humans pulling the strings all the time. all those times the market shut accidentally. or the market opens at the wrong time. or that time someone accidentally deletes all the GTC orders in order to "save some disk space". that time someone tests opening the market at the weekend and puts in the wrong date. Sometimes we are just trying to test that the things work and so we take awful risks like adding test orders, or failing over to test that backup versions of the trading infrastructure still work. All these things add human execution risk.

That said I find the US market structure is unfair Charles Schwab does protest too much. Retail orders never seem to get near the central order book. there is no direct market access. brokers just sell your order to whomever MM pays them for the spread in return for a kickback. this should be a fantastic fair multiplayer game, but instead its pay to win mobile crap with vested interests milking their customers.