A limit you write yourself breaks under load.
Two at onceTwo runs both read €1,000 left.KIFF locks draws per card: the second is refused.
RetryA timeout retries the same refund.KIFF keys the call: it counts once, and is sent once.
ScaleThe agent runs as three workers, or two agents share a Card.One balance, held centrally: no fresh allowance.
OutageThe balance cannot be read.KIFF refuses. Unknown is never read as zero.