Once I used a PassThrough stream instead of an EventEmitter. Apparently PassThrough streams retain some state as the data goes through them, and so eventually it caused a memory leak.
This memory leak was in a multi-machine process, which led to two processes thinking they were responsible for updating the database.
That caused mongo queries from the affected nodes to randomly execute after about an hour of being slammed from lack of memory.
Eventually the affected node would restart and start to function normally. Which was worse, because it allowed this problem to go unnoticed, until some users started reporting duplicate or corrupted data.
Stuff of nightmares.
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