Two kinds of failure, two fixes. Failing one step in twenty fails two thirds of the tasks: small errors multiply. Random failure / Systematic failure. How it trips. Sometimes, for different reasons. Always at the same step, in the same way. Retrying. Repairs it. Changes nothing. The fix. Automatic retry: 11 invoices become 28. Split the step in two, or take it out of the model. Switching models. Will not help. Will not help. The move: open the log and check whether the same failure always comes back at the same place.