About Cold Harbor, he allowed hundreds of his men to die.
The trenches were hot, dusty, and miserable, but conditions were worse between the lines, where thousands of wounded Federal soldiers suffered horribly without food, water, or medical assistance. Grant was reluctant to ask for a formal truce that would allow him to recover his wounded because that would be an acknowledgment he had lost the battle. He and Lee traded notes across the lines from June 5 to 7 without coming to an agreement, and when Grant formally requested a two-hour cessation of hostilities, it was too late for most of the unfortunate wounded, who were now bloated corpses. Grant was widely criticized in the Northern press for this lapse of judgment.
Not hundreds but thousands. The general history of Cold Harbor is the 7000 men died in the first 20 minutes. Many modern historians dispute this but there’s certainly contemporary reports to support the common wisdom. Yes, it was a bad call and Grant admitted it. Even in his memoirs he admitted this was his big regret of the the war - the last charge at Cold Harbor. But he only did it once whereas Lee made it repeatedly. And he certainly helped make up for it in the week that followed.