All the time. If you consider the number of people injured or killed every July 4th or New Year's Eve here in the US by people shooting pistols in the air, it shouldn't be surprising.

For example, when the Japanese attacked Pearl Harbor, some 200+ civilians were injured by fires thought to be caused by US AA fire. 32 civilians in Honolulu, which was not attacked by the Japanese planes, were killed by AA fire.

In the so-called "Battle of Los Angeles", a number of buildings were damaged by falling anti-aircraft rounds and shrapnel.

One report shows that the borough of West Ham in London suffered 1493 bomb hits during the whole of WWII, and 201 AA shells hit, with half exploding on impact. Note: PDF warning

So it's clear that it was quite common, just from those little snippets. I can only imagine what it must have been like in Berlin during WWII.