How it makes you pay more for a stock than your limit order works as follows. You see XYZ is trading for $49.99 and put in an order to buy 10,000 limit $50/share. Your trading platform replies you have bought 4000 at $50, the price has moved to $50.50 you have a 6000 share order waiting to be filled. You then say darn it I'll wait for it to go back to $50 and it goes up more and so on. As has happened to me many a time. Though as a trader you can play the game too and offer to sell shares at a high price if it looks like someone needs to buy. This stuff is one reason why paper trading gives a false sense of how easy trading is as historical prices don't actively try to figure what you're up to and screw you.