Never Underestimate it could cost you

Underestimating the opponent can lead to disaster. If you think that you are the better player your brain will go asleep. You lose concentration and focus. This leads to big blunders. One big blunder is usually enough to get a losing position.
Here's the thing; You can never be sure of the things you don't know. Your opponent may have something up their sleeve that you didn't see coming, no matter how weak they appear. By assuming your opponent could beat
you, you are better able to objectively think about all of the many ways in which they could potentially trump your argument or battle strategy, and prepare for those situations, no matter how unlikely they seem.

Never assume you have won until it's over, and you will have the best shot of doing so. If you underestimate your opponent, you give them the opportunity to see something you do not, and attack you in a way you are not prepared to defend yourself against.

quote from Mark Twain:

" There are some things that can beat smartness and foresight? Awkwardness and stupidity can. The best swordsman in the world doesn't need to fear the second best swordsman in the world; no, the person for him to be afraid of is some ignorant antagonist who has never had a sword in his hand before; he doesn't do the thing he ought to do, and so the expert isn't prepared for him; he does the thing he ought not to do; and often it catches the expert out and ends him on the spot."

"Threats are harder to see than captures. Some moves threaten checkmate, some threaten captures, some involve a general improvement in position. Some threats are crude, brutal, obvious. Others are unbelievably subtle in their intentions, refined in their execution. Some threats are sound and directed toward winning the game. Others are based on a foolish idea and will prove disastrous for the player who has devised them. Some threats are irresistible, others can be topped by a stronger threat.

In a game between good players, threats and counter-threats are essentially a matter of interplay of ideas and intentions. If each player does not always see through his opponent's threats, he is at least prepared for them. Thus, as in the case of captures, it is important to realize that threats are always possible, that they must be looked for. That is why threats are most dangerous when they are devised by an opponent who seems to have a lost game. When victory seems within your grasp is just the time when you are most likely to underestimate the other player's resources. "Simple" positions, too, are the downfall of many a player who feels that the game no longer requires careful scrutiny. Overconfidence is unquestionable the quality that leads many players to overlook their opponent's threats."

(Fred Reinfeld, 1910-1964)

“Do not underestimate the 'power of underestimation'. They can't stop you, if they don't see you coming.
― Izey Victoria Odiase
“There is no greater danger than underestimating your opponent.”
― Lao Tzu
"You and your opponent want the same thing. The only thing that matters is who works the hardest for it"
― Anonymous
"Never underestimate your opponent but never make him bigger than you."
― Philipp Schranz
"Know yourself, and know your opponent. They are never as strong as you think, and never as weak either.
― Carlos Newton
"The commonest mistake in history is underestimating your opponent; it happens at the poker table all the time."
― David M. Shoup
“Don't let expertise fool you into seeing false boundaries or underestimating those with wild dreams. "
― Naveen Jain
“A wise, right, and true estimate of one's own powers is necessary to their highest and best use. The general who overestimates his forces leads them to defeat; he who underestimates them does not lead them at all. "
― Lyman Abbott
"If you underestimate your opponent it could cost you when they surprise you"
― BourneRoot