For instance, an air raid beyond bomber's range where after completing the mission the pilots should land or eject at the enemy land and surrender.
During B-29 raids on Japan in 1944, in July and November, three B-29 bombers were forced to land in the Soviet Union either because of battle damage or fuel exhaustion after raids on Japanese positions in Manchuria. Because the Soviets were still not formally at war with Japan at this point, they interned the bombers' crews and happily accepted the windfall of the B-29s, a bomber they had unsuccessfully tried to get from the US.
Although these B-29 crews were not technically prisoners of an enemy, they were effectively out of the war. At this point in the Pacific Theater, becoming a prisoner of the Japanese was more than likely a death sentence, so internment in the Soviet Union was the only viable option to crews whose bombers could not return to their island bases.