Question

An SQS FIFO queue `account-commands` processes per-account commands (`MessageGroupId = account_id`) and guarantees in-order processing per account. At 10:00, overall throughput collapses: `ApproximateNumberOfMessagesVisible` climbs to 90k, but the worker fleet is mostly *idle* (low CPU, few in-flight). Dashboards show one message group is stuck — its handler keeps failing and the message is being retried (it's in flight, then back, repeatedly), and `maxReceiveCount` is high so it never DLQs. Recent context: one specific account triggered a command that hits a code path with a null-pointer bug deployed this morning. Explain why the *whole* queue's throughput collapsed and how you mitigate.