[Negative1]

Nope, that was possible, but the boolean operations could lead to numerical instability if you didn't align your brushes to the grid at a reasonable granularity, which could still lead to leaks. The flooding code was generally pretty well done but developers had to bandage issues like this in every Quake-engine game.

FYI Unreal still had leaks (which lead to major performance issues if uncontrolled, due to the PVS being too inclusive), but it was not as pronounced as the negative space still allowed the map to finish building the BSP (Quake just failed, sometimes without a clue as to why).