Rain overflow from balcony above me.
The upstairs balcony doesn't have a good way to drain water so it all ends up on the edge of my balcony. It hits my railing and then proceeds to spatter all over the place. I've thought about putting plastic sheets over the railing to deflect the water, but I'm not sure how I would prop up the sheeds, and it could get pretty expensive for an apartment dweller.
The rain overflow has flooded out my plants at times, especially the ones in self-watering containers that don't have any drain holes. I end up sitting out in the rain or after a storm trying to use a nail to make drainage holes. I ruined a pair of pants that way.
I never got around to drilling holes in the containers, because I don't want to drill when they are wet and potentially cause electrical problems.