This question comes up fairly regularly. You might try this answer from /u/astrogator, or this question, which attracted a lot of short answers two years ago.

I can't provide a complete answer but in The Guns at Last Light Rick Atkinson mentions numerous injuries (mostly bruises) among US troops based around the Ludendorff Bridge due to falling spent munitions from the incredibly dense concentration of anti-aircraft weapons protecting the bridge from German aircraft. Unfortunately, I'm away from home for several days and can't look up his sources.

Medium and heavy anti-aircraft guns generally used high explosive fragmenting ammunition so what fell back down would primarily be lighter splinters rather than an entire shell (however, duds did occur).