See https://screeps.com/a/#!/history/W73N64?t=17850234 - the bottom most spawn.

Players Icon & Vaejor were able to help me pinpoint the engine code:
https://github.com/screeps/engine/blob/master/src/game/structures.js#L1132
> This checks for if(this.spawning), which only checks the StructureSpawn, not the Creep.

https://github.com/screeps/engine/blob/master/src/processor/intents/spawns/renew-creep.js
> Same here

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I'm no engine expert, but the discussion we had suggested that the check should be done twice. First is structures.js to check via the API for creep.spawning and return ERR_BUSY. The other is in the renew intent, to check whether the creep actually spawning.
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I've lost many good honest creeps to this. Please think of the creeps and fix this soon!

Dewey