According to the Military Times, the midair collision of two F/A-18s from Naval Air Station Fallon last year was the result of a very simple pilot error:

Shortly after takeoff, they moved into a “wall formation” with the Super Hornets four abreast, putting 1.2 nautical miles between the two planes that eventually would collide. The lieutenant commander was flying one of the two inside planes.

“90-right, go,” the lieutenant commander announced, signaling everyone to turn.

While the three other planes turned right, for some reason the lieutenant commander turned left. About 11 seconds later, the side of his plane collided with the lieutenant’s at 792 knots.

Words have meaning.  This was a night sortie, meaning there were very few visual cues, forcing the flight to rely on verbal communication for some maneuvers.

The flight lead said right.  Then he went left.  He wouldn’t be the first person to mean his military right.

Both pilots survived, though the wingman ejected while the flight lead was able to land.