Already mentioned with Snell senario, but pulling starters based on pre-game strategy. Just use your eyeballs and not the computer sheets. It's not that hard to tell when a guy us dominating.
Starting the prevent defense (or prevent victory) too early in a football game.I have seen so many games where a teams defense is dominating by putting pressure on the qb. Some coaches start the prevent too early and it allows the other team to zip down the field and either tie it up or outright win the game.
Lastly, McNamara keeping Buckner in the game against the Mets, even though he had put Stapleton in the game for defense the entire second half of the year. Just wanted Buckner, after a long good career, to be on the field when they clinched.