In the confusion of battle, it was not unknown for troops to fire on their own side by mistake. The American military referred to this as friendly fire, as distinct from enemy fire. Since there is nothing friendly about it, friendly fire resulting in casualties has sometimes been called fratricide. Commonwealth troops sometimes referred to it as own fire or self fire. The term blue on blue dates from long after the time frame of the Pacific War.
Certain combat activities were particularly prone to friendly fire incidents. When patrolling in dense jungle, it was difficult for infantrymen to keep track of the members of the patrol. Contact with the enemy usually led to fierce firefights, and in the confusion it was easy to fire by mistake at one's own man, or to fire at an enemy but hit friendlies who had blundered into the line of fire. Night operations suffered from the same dangers. Because of the Japanese fondness for night operations, most Allied troops adopted a policy of staying in their foxholes at night and assuming anything up and moving around was the enemy. The U.S. Marines used passwords containing the letter 'L' (which was difficult for most Japanese to pronounce) to help identify their own in the dark.
Long-range weapons also posed a risk of friendly fire. Artillery had to be careful not to drop short rounds on friendly troops. When in doubt, the artillerists fired long. In some cases, artillery was deliberately called down on friendly positions that were being overrun, on the theory that the defending troops were dug in and would take less punishment than the attackers. Often the forward observer himself made this difficult decision, calling down fire on his own position, as happened during the night fighting around Henderson Field during the Guadalcanal campaign.
Ground support led to many friendly-fire incidents. The aircraft flying ground support had only seconds to identify their target, drop their munitions, and get clear to avoid return antiaircraft fire. It is no surprise that mistakes happened, even when ground forces were careful to mark the enemy positions with smoke. Such incidents became less common as the war progressed, but they were never entirely eliminated.
It was not just ground forces that suffered from friendly fire. There were numerous incidents of naval forces firing on their own ships by mistake during night actions, as well as situations when a naval force hesitated and lost a tactical opportunity from fear of firing on its own ships. During the Naval Battle of Guadalcanal, both situations occurred in the same battle.
Ships also suffered from friendly fire from aircraft, which often had difficulty correctly identifying ships from high altitude. Submarines seemed particularly prone to this kind of attack early in the war, but even surface task forces were sometimes attacked by their own side, as occurred during the Battle of the Coral Sea, when an Allied cruiser force was attacked by American land-based bombers (fortunately without any casualties.)
Aircraft caught it in return. Fighters had to be careful about engaging enemy aircraft close to their own forces, whose antiaircraft gunners often fired on both aircraft with fine impartiality. Early in the war, many Allied aircraft had identifying insignia containing red circles; these were quickly changed so that they could not be mistaken for the distinctive Japanese hinomaru or Rising Sun disk. Aircraft also took friendly-fire casualties from each other, particularly at night. There were incidents where aircraft on low-level attacks followed too closely behind each other and an aircraft was caught in the explosions from the preceding plane's bombs.
Avoiding friendly fire was largely a matter of training. Lax fire discipline among green troops was evident in the New Georgia campaign and accounted for 12 percent of all casualties. Sadly, a disproportionate share of these were fatal, since most friendly-fire casualties were inflicted at close range with rifles or machine guns, which are particularly lethal. Bergerud quotes one infantryman:
Dysentery was rampant on Guadalcanal. It is a miserable malady ... The desire to relieve yourself is just tremendous. At night, what do you do? We had passwords, but the Japs were all over and guys were quick to shoot. So do you stay in the hole or go out for a minute and risk getting shot. Those were the alternatives. Many people stayed in their hole, but I'm afraid many of the men shot after dark had their pants down. It was amazing how many ways you could get hurt in World War II.
References
The Pacific War Online Encyclopedia (c) 2007-2008 by Kent G. Budge. Index