What they knew
After Ligny, Napoleon sent Grouchy to follow the retreating Prussians and keep them away from Wellington. His orders were not precise, and Napoleon's later messages arrived slowly.
On the morning of 18 June, over breakfast, Grouchy and his officers heard heavy cannon fire to the west.
The choice
General Étienne Gérard urged him to march toward the guns. Grouchy refused: his orders were to pursue the Prussians, and he believed they were in front of him at Wavre. In fact most of the Prussian army was already marching to join Wellington, leaving a rearguard to hold Grouchy.
The consequences
Grouchy fought and won a minor action at Wavre while the main Prussian army helped decide Waterloo. Whether he could have arrived in time over muddy roads is still debated. Napoleon later blamed him. Many historians put more weight on the vagueness of Napoleon's own orders.