Making the same mistake again and again may turn into a bad habit, but a strong player would punish you for it and show you clearly that it's a mistake.

Playing against weaker players exclusively may allow you to regularly make mistakes that a stronger player would punish consistently and clearly. Always being allowed to make such a mistake without punishment may give you the incorrect impression that it's a perfectly fine move and can turn into bad habit that is hard to get rid off.

Playing against weaker players exclusively may allow you to regularly make mistakes that a stronger player would punish consistently and clearly. Always being allowed to make such a mistake without punishment may give you the incorrect impression that it's a perfectly fine move and can turn into bad habit that is hard to get rid off. Some examples of such habits:

For some, the answer to this may be obvious. But others may think that only playing against weaker opponents or only occasionally playing against equally strong opponents is fine. If you want to become a stronger Go player, I think it is not sufficient. You should always try to play against players who are significantly stronger if possible. Players much weaker than you will likely not punish you for many of the mistakes you make or punish you incorrectly. If that happens often enough, it might give you the impression that these mistakes are actually perfectly fine moves. Over time, this might develop into a bad habit that is hard to get rid off later. But if your opponent consistently punishes the same kind of mistake in the same way, then it is pretty hard to miss that you made that mistake. Eventually that should allow you to understand what the mistake is and how to possibly avoid it. It is just an example of the power of human pattern recognition. Of course this also indicates that your games shouldn't be too far apart in time or recognizing the pattern becomes hard. A couple of example mistakes that are easy to make and easy to turn into a habit and that are hard to punish if you're not already quite experienced:

• Playing the same opening every time.
• Playing incorrect openings.
• Avoiding Kou fights like the plague.
• Ending Kou fights prematurely.
• Playing Kou "threats" that are not sente.
• Playing very early invasions.
• Starting a new weak group when you already have several.
• Avoiding invasions completely.
• Invading too late.
• Not defending your groups properly (or at all).
• Always answering your opponent's move locally and defensively, never taking the initiative.
• Constant overplays.
• Always playing bad shape.
• Allowing your opponent too much influence.
• Trying too hard to gain a lot of influence yourself, giving away territory.
• Slacking off after having created a lead. In general, underestimating an opponent based on a past bad performance.
• Completely ignoring the endgame.

• Playing the same opening every time.
• Playing incorrect openings.
• Avoiding Kou fights like the plague.
• Ending Kou fights prematurely.
• Playing Kou "threats" that are not sente.
• Playing very early invasions.
• Starting a new weak group when you already have several.
• Avoiding invasions completely.
• Invading too late.
• Not defending your groups properly (or at all).
• Always answering your opponent's move locally and defensively, never taking the initiative.
• Constant overplays.
• Always playing bad shape.
• Allowing your opponent too much influence.
• Trying too hard to gain a lot of influence yourself, giving away territory.
• Slacking off after having created a lead. In general, underestimating an opponent based on a past bad performance.
• Completely ignoring the endgame.

Similar things of course apply in Chess, but I'm not a Chess expert, so I would have to seek outside expertise.

Apart from playing against strong opponents, this list should also help you understand why playing against a wide variety of opponents is useful. Most players are not equally strong in all aspects of the game and thus some opponents may be able to punish only certain kinds of mistakes, while other players punish other mistakes. Playing against the same opponent all the time, even if strong, might leave blind spots. I can remember some of my opponents and certain strengths and weaknesses they appeared to have (and that I didn't know about before, of course):

• One opponent was really strong for 30 minutes but then started to make several silly mistakes. He was quite young and probably couldn't concentrate for a long time.
• One opponent won the game, but just didn't count, despite having plenty of time. He made an unnecessary defensive move, also without using time, and consequently lost by half a point.
• One opponent was clearly stronger and more experienced than me and held a pretty steady lead of almost 20 points until the end of the game. Then he made a quick and careless move when the game was almost over, allowing me to turn the game around with a relatively obvious reply. He apparently had stopped taking me seriously or was already exhausted. I had waited for that specific mistake for probably 50 moves as it was my only hope.
• One opponent started a complicated Jouseki apparently without knowing how it works and made a catastrophic mistake that gave me a lead of almost 40 points very early in the game. I expected him to resign, but he didn't. He kept playing decent moves, using all of his time, and I grew increasingly frustrated. Eventually I overlooked one thing and he was able to turn the game around. Essentially the reverse of the previous entry.
• One opponent loves to play very early invasions, which usually ends with him having to fight for his life and often resignation.
• One opponent played very amateurish and simple to refute openings, but played the middle game on a much higher level, clawing back all the advantage I had created.
• Another opponent also doesn't like to spend any time thinking about the opening, plays the middle game decently, but really starts looking for all kinds of weaknesses in your position in the endgame. And usually finds one, stressing you out when you thought the game is already over.
• More than one opponent has given away 20 to 30 points in the endgame without me having to push for that and apparently without being aware of it.
• Several opponents, especially inexperienced ones, like to avoid Kou at all costs or end a Kou fight as quickly as possible.
• Several opponents, also inexperienced ones, like to abandon groups that are in danger, but clearly have survival chances.
• One opponent played really defensively, even defending groups that already have 2 obvious eyes.

Avoiding all of these habits and becoming a well-rounded player (no, not a chubby player) is probably worth 1 or 2 stones of strength by itself. So, if you think you have one very clear weakness, work on it.