I am very skeptical about the honesty of retail brokers. I used to day trade for profit all the time, but it has become impossible in the last five years. Here are things I have seen:
Crazy outside fills on buy or sell orders - even on limit orders. On Schwab I started using market on options because 80% of the time I would get price improvement over the bid/ask- BUT, 20% of the time they would fill me at far below even all the quotes I had seen go by during the time my trade was open. Sometimes my trade would open at -$750 for example.
I have seen stocks that have not had a down-tick in two hours suddenly reverse and lose a TON of gains the the second I bought in. Even in stocks that report 2-million shares in volume that day. There is no logical reason for this, except "bid-shilling" where they are trolling for a buyer who takes the bait and then they immediately start shrinking the value of your position. And most of the time cannot wait them out, the longer you hold on the more you bleed, and then then the second you get out, it turns around.
Definitely not filling immediately, and while your order is open the stock moves up 2 points and then you see your fill come in at the price after the move (so you get no gains).
To sum up, I have seen all of these things, and often several times a day. I truly do believe there is illegal action going on at brokerage houses these days, and I would like to see it blown wide open.
What have you seen?