user13147: Back when I was in high school, I played chess with a group of other students. One of the students would pick on me sometimes. Once, we decided to play and he cornered me into a forced mate in two. Before the threat turned into the final execution however, I swept my arm across the chessboard vehemently, blasting the pieces across the room. Needless to say, he got very mad. I spontaneously kept doing this for a few more games (to my sole entertainment).
Curt von Bardeleben is known as the opponent who never returned. After move 25 in the Battle of Hastings, Steinitz's opponent mysteriously disappeared:

At this point, Steinitz's opponent left the tournament hall and never reappeared. By these offensive means he hoped to deprive Steinitz of 'a piece of immortality', ... - Art of Attack, V. Vukovic. pp 21

What are some of the most unsportsmanlike manners witnessed in over-the-board chess play?

user15223: Three examples listed by James Eade in Chess for Dummies (excerpt here):

Alexander Alekhine once resigned a game by picking up his king and hurling it across the room. (This incident is also mentioned by Edward Winter.)
Aron Nimzowitsch once lost a game which cost him first place in the tournament, and apparently got up on a table and shouted: "Why must I lose to this idiot?" The game is here, you can see various remarks about it in the comments section.
An unnamed Danish player once lost his queen after a finger slip. That night, he slipped back into the tournament hall and cut the heads off all the queens.


user20776: Trash talking during a game and calling the opponent fish or patzer.  Usually only seen during casual games with relatively fast time controls.  
I have seen it with IMs and GMs doing it , usually I think in 'fun' without an evil intent, but sometimes it is definitely meant to win unfairly especially when they  have money on the game. 

user7534: "Blackburne made an insulting remark, Steinitz spat towards him – though not necessarily hitting him – and Blackburne smashed him in the face with his fist. It happened at the City of London Chess Club." (From Edward Winter's excellent Chesshistory.com site)

user7147: In a game I was personally playing in, my opponent was a first-time tournament attendee.  It was an early round and I would say the position on the board was about equal.  But my opponent made a blunder that left him open to a mate-in-one, which I found.  He got angry, and used his arm to sweep the pieces off the board, on to the floor.  Then he stormed out of the tournament hall.  He did not come back for the next round, I don't think he ever played another tournament game again.
I've seen (as a spectator) a well known local player in our city get upset with himself during a game because he blundered in a good position - he picked up the clock and threw it hard against a wall, breaking it.  Fortunately (sort of...), it was his own clock.

user12084: There is a book on the topic.  "Underhanded Chess: A Hilarious Handbook of Devious Diversions and Stratagems for Winning at Chess", by Jerry Sohl.  It was originally published in the '70s, but apparently has been picked up again by a publisher and is available from on-line booksellers.  Bobby Fischer, naturally, gets a lot of ink from the author.

user-1: Being rated, 1700 I played against a 1880 and he lost essential material. He shot down all the pieces and they fell on the floor. 
We were playing om four boards, it was a club match, and my club partners and opponents were amazed because of the noise. 
After that, we were winning the match 2-1. It was last round and we were virtually classified for next league, while the club of this guy was not going to promote to next league because they needed to beat us at the match.
The last game ended on a blunder of the player of the club of my angry opponent, so we won 3-1, classifying ourselves. 
He started to shout to his club partner: "How can you play that?"
I felt a lot of embarrassment. He had the eyes 
flushed. 
I go to tournaments for fun and because chess is good for my brain; I don't understand why some people take an amateur sport so seriously.

user-1: Here are some examples of unsporting behaviour I have heard of before:

Kicking opponent under the table (from Josh Waitzkin in the Chessmaster tutorials)
(Added for interest) Running down the clock in correspondence chess with the aim of waiting for an opponent to die of old age (this shouldn't be a major issue under the ICCF, as games with deceased players are now cancelled or substitutes are called, but they could be misrecorded as a loss on time)
Using an iPhone for analysis (hat tip to @SmallChess)


13147: <p>Back when I was in high school, I played chess with a group of other students. One of the students would pick on me sometimes. Once, we decided to play and he cornered me into a forced mate in two. Before the threat turned into the final execution however, I swept my arm across the chessboard vehemently, blasting the pieces across the room. Needless to say, he got very mad. I spontaneously kept doing this for a few more games (to my sole entertainment).</p>

<p>Curt von Bardeleben is known as the opponent who never returned. After move 25 in the <a href="http://www.chessgames.com/perl/chessgame?gid=1132699" rel="noreferrer">Battle of Hastings</a>, Steinitz's opponent mysteriously disappeared:</p>

<blockquote>
  <p>At this point, Steinitz's opponent left the tournament hall and never reappeared. By these offensive means he hoped to deprive Steinitz of 'a piece of immortality', ... - Art of Attack, V. Vukovic. pp 21</p>
</blockquote>

<p>What are some of the most unsportsmanlike manners witnessed in over-the-board chess play?</p>
 15223: <p>Three examples listed by James Eade in <em>Chess for Dummies</em> (<a href="https://www.chess.com/forum/view/general/the-worst-losers-in-chess-history" rel="nofollow noreferrer">excerpt here</a>):</p>

<ol>
<li><p>Alexander Alekhine once resigned a game by picking up his king and hurling it across the room. (This incident is also mentioned by <a href="http://www.chesshistory.com/winter//extra/violence.html" rel="nofollow noreferrer">Edward Winter</a>.)</p></li>
<li><p>Aron Nimzowitsch once lost a game which cost him first place in the tournament, and apparently got up on a table and shouted: "Why must I lose to this idiot?" The game is <a href="https://www.chessgames.com/perl/chessgame?gid=1007226" rel="nofollow noreferrer">here</a>, you can see various remarks about it in the comments section.</p></li>
<li><p>An unnamed Danish player once lost his queen after a finger slip. That night, he slipped back into the tournament hall and cut the heads off all the queens.</p></li>
</ol>
 20776: <p>Trash talking during a game and calling the opponent fish or patzer.  Usually only seen during casual games with relatively fast time controls.  </p>

<p>I have seen it with IMs and GMs doing it , usually I think in 'fun' without an evil intent, but sometimes it is definitely meant to win unfairly especially when they  have money on the game. </p>
 7534: <p>"Blackburne made an insulting remark, Steinitz spat towards him – though not necessarily hitting him – and Blackburne smashed him in the face with his fist. It happened at the City of London Chess Club." (From Edward Winter's excellent <a href="http://www.chesshistory.com/winter/extra/violence.html" rel="nofollow noreferrer">Chesshistory.com</a> site)</p>
 7147: <p>In a game I was personally playing in, my opponent was a first-time tournament attendee.  It was an early round and I would say the position on the board was about equal.  But my opponent made a blunder that left him open to a mate-in-one, which I found.  He got angry, and used his arm to sweep the pieces off the board, on to the floor.  Then he stormed out of the tournament hall.  He did not come back for the next round, I don't think he ever played another tournament game again.</p>

<p>I've seen (as a spectator) a well known local player in our city get upset with himself during a game because he blundered in a good position - he picked up the clock and threw it hard against a wall, breaking it.  Fortunately (sort of...), it was his own clock.</p>
 12084: <p>There is a book on the topic.  "Underhanded Chess: A Hilarious Handbook of Devious Diversions and Stratagems for Winning at Chess", by Jerry Sohl.  It was originally published in the '70s, but apparently has been picked up again by a publisher and is available from on-line booksellers.  Bobby Fischer, naturally, gets a lot of ink from the author.</p>
 -1: <p>Being rated, 1700 I played against a 1880 and he lost essential material. He shot down all the pieces and they fell on the floor. </p>

<p>We were playing om four boards, it was a club match, and my club partners and opponents were amazed because of the noise. </p>

<p>After that, we were winning the match 2-1. It was last round and we were virtually classified for next league, while the club of this guy was not going to promote to next league because they needed to beat us at the match.</p>

<p>The last game ended on a blunder of the player of the club of my angry opponent, so we won 3-1, classifying ourselves. </p>

<p>He started to shout to his club partner: "How can you play that?"</p>

<p>I felt a lot of embarrassment. He had the eyes 
flushed. </p>

<p>I go to tournaments for fun and because chess is good for my brain; I don't understand why some people take an amateur sport so seriously.</p>
 -1: <p>Here are some examples of unsporting behaviour I have heard of before:</p>

<ul>
<li>Kicking opponent under the table (from Josh Waitzkin in the Chessmaster tutorials)</li>
<li>(Added for interest) Running down the clock in correspondence chess with the aim of waiting for an opponent to die of old age (this shouldn't be a major issue under the ICCF, as games with deceased players are now cancelled or substitutes are called, but they could be misrecorded as a loss on time)</li>
<li>Using an iPhone for analysis (hat tip to @SmallChess)</li>
</ul>
