Must have been one of those sharp, square-edged water balloons. Check out the crease it made...

Glad it didn't hit a person! Could have been deadly. Or caused a concussion at the very least. Should buff right out of the VX though...

I dunno... I threw many a water balloon into a parking lot from a 4th floor balcony - hit many cars while aiming for the people hiding behind them - and never saw one cause any damage whatsoever. Could they have tossed something else and followed it up with a water balloon?