
NO stealing when up by 10 or more runs:

This is a rule that is seen in essence throughout many other popular sports leagues. When a team is having an off night and losing by many runs or points, you don’t keep scoring, because at that point you are just humiliating the other team. Stealing is seen as an aggressive, risky play in order to try to gain a run that you otherwise wouldn’t have gotten, and doing so when you are up by many runs can lead to retaliation in the form of hit-by-pitches. Yet, as modern teams begin to score more and more runs, no lead seems safe. This is where ambiguity and logic come into play: How big does your lead have to be before you stop trying? And why is it ok for one team to keep trying while the other has to play conservatively?
NO stepping on the mound (A-Rod stepping on mound v. A’s in 2010, Dallas Braden pitching)

The mound is a pitcher’s safe haven. This domain being only for pitchers makes it all the more disrespectful when a batter gets bested, and then proceeds to run across the mound on his way back to the dugout. This occurred in 2010, when New York Yankee Alex Rodriguez ran across the mound of Oakland Athletics pitcher Dallas Braden. Typically, this action results in the pitcher and batter exchanging words, and then someone on the batter’s team getting thrown at.
NO pimping home runs (Jose Bautista 2015 ALDS v. Rangers)

Respecting the opposing pitcher and not overly admiring home runs have been longstanding unwritten rules. Perhaps no player is as iconic for disregarding these rules as former Toronto Blue Jays outfielder Jose Bautista, who launched his bat several feet into the air after blasting a 3-run homer against the Texas Rangers in the 2015 playoffs. This move received heavy criticism from the Rangers and traditional fans alike, and resulted in multiple bench clearings and fights between the teams in coming seasons.
NO bunting to break up a no hitter (Erik Aybar (Angels) v. Justin Verlander (Tigers)2011)

Attempting to lay down a bunt to intentionally break up a no-hitter is a highly frowned upon act in the baseball community. This unwritten rule was most notably challenged in a 2011 game between the Los Angeles Angels and the Detroit Tigers, where Angels shortstop Erik Aybar tried to drop a bunt down in the 7th inning to ruin Justin Verlander’s bid for a no-hitter. Verlander was upset at this supposed “rule breakage,” calling the play a “bush league” move by Aybar.
NO swinging 3-0 when up by a lot of runs (Tatis v. Rangers 2020)

Running up the score is a sign of bad sportsmanship and disrespect toward the opposing team. In August of 2020, San Diego Padres shortstop Fernando Tatis Jr. hit a grandslam with a 3-0 count to put his team up 14-3 on the Texas Rangers in the eighth inning. After the game, both coaches and fans expressed that they felt this was disrespectful and unnecessary.