I think you are thinking about the commentary below. This example was for banks who do not offer an opt-in and only authorize on positive balances.
If you authorize into overdraft and have an opt-in, you can charge a fee, provided your disclosure accurately describes your practice.
Commentary to 1005.17(b) Opt-In Requirement
iv. Application of fee prohibition.
Assume an institution does not provide an opt-in notice, but authorizes an ATM or one-time debit card transaction on the reasonable belief that the consumer has sufficient funds in the account to cover the transaction. If, at settlement, the consumer has insufficient funds in the account (for example, due to intervening transactions that post to the consumer's account), the institution is not permitted to assess an overdraft fee or charge for paying that transaction.
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I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.