Friendly fire has been a part of war at least as far back as the American Civil War (when Stonewall Jackson was killed by one of his own troops) and probably even earlier than that. If you read about or watch the movie "Lost Battalion", you'll find that friendly fire happened in WWI. It was much more common in WWII and took many forms, such as artillery, ships, or planes accidentally killing friendly forces or anti-aircraft artillery shooting down our own planes.* There were several "friendly fire" incidents during the breakout of Normandy where heavy bombers were used for close air support. An estimated 10% of the bombs were dropped off target, many of them ended up falling on Allied forces. On July 25, 1944 alone, about 500 troops were killed by friendly fire in a single incident. It was a tragic but expected part of war.

Friendly fire deaths continued through the Korean and Vietnam wars as well. It continues to this day and there's no end in sight.

* From a book on British air defenses by the head of their home defense, "It is said that the British anti-aircraft gunner can recognize three types of planes: approaching and considered hostile, receeding and considered friendly, and Lysanders."

(A Lysander was so distinctively ugly that it was hard to mistake it for anything else.)