The ACH rules state that a debit should not be posted to an account before settlement, however, credits may be posted before settlement. This becomes an issue if a debit or credit comes in on a Saturday, for example, but the settlement does not occur until Monday. Some systems memo post the debit or credit when it occurs (before settlement), which in the case of a debit, is contrary to the ACH rules. As to how often this happens, I've heard it is not an uncommon practice. Apparently, this is often a result of processing systems limitations, rather than a conscious choice to post in this manner.
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--A bad day at sea is better than a good day at work.