Correcting errors on previous returns

Found an error? Is a penalty likely? This article explains which penaty regime will apply to mistakes you find, and how to put them right.
It is not uncommon that when preparing a tax return the agent discovers errors that have been made on a previous return. This may be the previous self assessment return, or errors made on VAT returns submitted by clients during the year.
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"it depends"
I think that if the error is considered careless then a penalty can still apply if the tax increases. The tax due should have been paid by 31st January, you amend return in say July, there is a late payment of tax and possible penalty.
I hope that these kinds of things will end up in a nil penalty. Let's say you amend a return but make no separate disclosure, if HMRC come along 6 months later and say it is a careless mistake do we expect a nil penalty? It's unprompted, the tax underpaid paid and interest charged. Surely there will not be a penalty in these cases, unless it is a deliberate error and in those cases if deliberate surely someone will not then decide to amend the return anyway!
Suitable wording for disclosure notes
Does anybody know where to find this? The article mentions them in the last sentence but I am unable to locate where to download them from.



Amendments to Self Assessment Tax Returns
Am I right in thinking that, if the mistake is found within 12 months following the 31st January after the tax year then an amendment can still be made to the Tax Return? If so, is this without penalty?