> ## Documentation Index
> Fetch the complete documentation index at: https://docs.methodfi.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Payment Errors and Constraints

> Processing windows, posting timing, return codes, and idempotency.

## Payment Errors and Constraints

* Payments submitted outside of processing windows will be held until the next window.
* Posting timing depends on the creditor and is outside Method's control.
* Return codes may be provided when a payment fails or is returned.
* Duplicate payments can be prevented using [idempotency keys](/reference/idempotency).