Changing the plan or trying to increase your position in the middle of the trade is one thing that can bring about unfavourable result especially when its basically to increase your profits because the trade is playing out as predicted.
One thing is not having the right entering point from your analysis and along the line, you choose to change. It’s as deadly as it sounds because, I trend is supposed to last for just a good enough time and then it changes. Now, haven’t identified the direction of a trend and it happens to not favour your analysis and you choose to change your trading position based on the current trend’s direction, you could as well experience a change in trend direction and that would be more damaging than having maintained your earliest analysis.