It’s amazing how the actions of a select few affected the battle so dramatically:
Chamberlin: see above.
John Buford’s retreat to the high ground west of Gettysburg, thus securing the tactical advantage for the Union.
Richard Ewell’s reluctance to continue the battle on the evening of July 1st, possibly costing the Confederates a chance to route the Union on day 1.
Dan “dumbass” Sickles’ moving his corps ahead of Union lines, creating a salient and almost causing disaster for the Union on day 2.
James Longstreet telling Lee not to perform Pickett’s charge, Lee going ahead anyway.
Custer not stopping Stuart in the East. Had Lee gotten the coordinated attack he expected, the Union lines would have panicked and fled.