The things in geostationary orbit don't crash into each other, they just constitute a relatively high density of "stuff" all in one place that non-geostationary objects at the same altitude could hit every time they cross the equator.
The worst case would presumably be something large and heavy put into orbit around the equator at the same altitude and speed as geostationary orbit but going in the opposite direction.
The worst case would presumably be something large and heavy put into orbit around the equator at the same altitude and speed as geostationary orbit but going in the opposite direction.