This is a mistake that can be corrected in the next year, because it's a mistake that could have been anticipated from the start by setting a stop loss. This is a common mistake that many people make.
I have also experienced mistakes like this until I finally realized that in the future installing stop loses is something that I should not miss. And I have this kind of thinking and apply it to myself, when I make the same mistake then it is no longer a mistake, but a foolishness.
The trivial thing about this is he has a very good run on 2 quarters already while he is not applying stop loss that time. Over confidence for not applying stop loss after winning is something that a human error can repeatedly do when they are on the winning streak.
Stop loss should be applied at all time and I guess this lesson learned this year will be applied properly next year.
Stop loss is now being ignored by some trader by using cross position and applying huge capital on a small trade for a better liquidation price.
I can understand that, when we always succeed without setting stoplos then at that time our confidence will increase dramatically and this is where the mistake lies. We should be able to control our minds so that we don't feel that what we do will always work, because the market can change at any time and this will cause us to experience losses that are even greater than the profits we get.
I'm sure he is knowledgeable about trading, but then again the market can crash unexpectedly and it can happen very quickly. So to anticipate that then in the future we should put a stop los.