It will happen for all mobs that spawn a rider with it. The entity spawned has `onInitialSpawn` (MCP name) called, which for the strider and spider has a chance of spawning a strider.
Later the spawner minecart adds all passengers of the entity it wants to spawn to the world, but since the spider and strider have already done that it will cause a crash later in the tick when the entities are processed.
suppergerrie2
The same thing happens for striders.
It will happen for all mobs that spawn a rider with it. The entity spawned has `onInitialSpawn` (MCP name) called, which for the strider and spider has a chance of spawning a strider.
Later the spawner minecart adds all passengers of the entity it wants to spawn to the world, but since the spider and strider have already done that it will cause a crash later in the tick when the entities are processed.