Here are four mistakes and their effects on the Gettysburg battle:
Confederate mistakes:
1. Pickett's Charge - Gen. Lee ordering the massive frontal assault on the Union center on the third day was costly and ineffective. It resulted in massive Confederate casualties and failed to break the Union lines. This was arguably the best chance for the Confederates to win the battle and they squandered it.
2. Failure to attack on July 2 - The Confederates had success on July 1 in driving back the Union troops but failed to capitalize on it by launching a full attack on July 2. This allowed the Union troops to regroup and strengthen their line on Cemetery Ridge.
Union mistakes:
1. Poor initial position - The Union army's initial position on the low ground south of Gettysburg was poor strategically. They were fortunate the Confederates did not launch a full attack on July 1, allowing the Union troops to withdraw to the heights on Cemetery Ridge.
2. Failure to pursue - After repelling Pickett's Charge on July 3, the Union troops failed to immediately counterattack and pursue the retreating Confederates. This allowed most of Lee's army to escape instead of being destroyed.
In summary, the Confederate mistakes resulted in missed opportunities to potentially win the battle while the Union mistakes almost cost them the battle initially but were ultimately overcome through solid defensive maneuvers on Cemetery Ridge.