Don't drop poison messages on the floor — keep them for later.
When a queue overflows, the simplest behavior is to drop new arrivals. But silent drops are a debugging nightmare: you can't see what was lost, can't replay it, and may not even know it happened.
A dead-letter queue is a side-channel for messages that couldn't be processed. Overflows, poison pills, repeated failures — they all land here instead of being lost. Operators inspect, fix, and replay them.
In Flow, draw an edge from a queue to a target node, click the edge, and tick "Dead-letter queue". When the queue overflows, those messages land at the DLQ target instead of vanishing.
Heavy write load to a slow consumer overflows the queue and you can't tell what was lost. Wire a dead-letter queue from the queue's overflow path to a database for later inspection.