From Bailey’s magisterial Field Artillery and Firepower:
At the Battle of Waterloo (18 June 1815) Captain Mercer’s British battery had repulsed a French cavalry charge when it was engaged by advancing Prussian guns, suffering serious casualties. Mercer’s guns replied but were saved by a Belgian battery (described as “beastly drunk and not at all particular to which way they fired”), which assumed the Prussians to be French and shattered them with shot and canister.
The British, Prussians, and Belgians were all theoretically on the same side. Which somehow won.
His Majesty: “Friendly fire isn’t very.”