Friendly fire has probably happened since people began shooting at one another. It's a serious problem with no easy answers, as the British have found.

Friendly fire certainly dates back at least as far as the American Civil War, when Stonewall Jackson was killed by one of his own units.

It certainly happened during WWI, as in the case of The Lost Battalion.

With the advent and implementation of maneuver warfare, the problem just gets harder. According to this source, the percentages of American casualties due to friendly fire ranged from 1.5% to 2.85% during WWII, Korea, and Vietnam, mostly from aircraft. It hit over 17% during Desert Storm, mostly from ground fire, but the small number of total casualties tend to skew the statistics.

I saw an article yesterday about how the investigation into a friendly fire incident where some British soldiers were killed may have been caused by the US pilots taking speed to stay awake during very long missions. I can't find a link to the article today, though.

As for the AC-130s, they really aren't that loud and have IR suppression to make them harder to shoot down. They operate low and slow, making them vulnerable in daylight in the face of air defenses. During Desert Storm, "Spirit 03", an AC-130H was shot down shortly after sunrise when it stayed to provide support to ground forces under attack. All 14 crewmembers on board were killed.

Given the sensors on board AC-130s, I honestly don't know how they could've mistaken a few pot-shots being fired into the air for hostile gunfire being directed at them. I'm pretty sure there were recorders on those sensors, so it would seem a relatively simple matter to know what the crews saw at the time. They probably wouldn't want to release those tapes to the general public because it would reveal the sensor system's capabilities, but the tapes could be screened by investigators from the military and shown to Afghan representatives as well.