Many custom maps are created without info_observer_points (those various cameras you look through in spec mode), and as soon as a player dies, or no-one is left alive on your team, the server crashes.
This may be because the game tries to give those players waiting to respawn, an info_observer_point to look through.
Unsure about other mods, but it's a problem in TF2.
This plugin adds a random artificial info_observer_point, if the map doesn't have one. It may not be pointed in the right direction, but it will prevent server crashes.
Installation- Copy sm_observerpoint.smx to addons/sourcemod/plugins