I've been reading a bit about the sheer volume of anti-aircraft fire used in WWII and became curious about this. Occasionally one hears about bullets fired into the air during celebrations falling back down at high speed and hitting people or property; given the massive amount of rounds put into the air during WWII, was accidental friendly fire a fear of people living near the batteries? Are there any records of civilian casualties as a result of spent anti-aircraft ammunition?
Yes, it certainly happened in WWII, although it would be impossible to get a quantitative analysis to that degree. Certainly there are plenty of examples to illustrate it though. In this old thread /u/bigglesworth_ and I discuss falling ordinance which you might find interesting.