MB4 209a: Disposing of Wastewater on Shabbos in a Yard (357:1 - part 1)

Passively allowing wastewater to leave a reshut hayachid into a reshut harabim on Shabbos.

If a yard is 4x4 amah or less and is adjacent to reshut harabim you can't pour wastewater into the yard on Shabbos because it will flow into reshut harabim unless there is a hole large enough to hold 2 סאה in the reshut hayachid.