Due to a misunderstood, or poorly written, order by General Grant
at the Battle of Shiloh, Union General Lew Wallace and his division found themselves behind the Confederate troops, a near-perfect position to attack from. Unfortunately, Wallace then got a message from General Grant, asking why he wasn't at Pittsburg Landing, with the rest of the army. Wallace could have attacked right then, and been a hero, but instead chose to take his division back around, marching for several hours, trying to meet the Union Army at Pittsburg Landing, finally arriving around 7pm, after 13 hours of marching, when the battle was nearly over for the day. To say that 'Grant was not pleased' probably understates the case, massively.
So, suppose that second messenger didn't find Wallace in time, or that Wallace decided that going all the way around like was a terribly bad idea. Either way, Wallace attacks the advancing Confederates from the rear, likely achieving a fair bit of surprise.
So, does this end the battle sooner?
Does it reduce the overall Union casualties?
What effect does it have on the rest of the war?