12.31The hardest payment failure

Sequence:

Mule claims PAY-123
 ↓
payment provider charges card
 ↓
Mule crashes before storing result

Now idempotency record says maybe:

PROCESSING

and you don't know whether payment succeeded.

You need reconciliation with provider using:

provider transaction ID
idempotency key
query transaction status

This demonstrates:

Idempotency design must include recovery from ambiguous intermediate states.