Stroppy Author's stroppy tour is over. After stropping at the health authorities in CrowAshire, and then at the NHS, I believe my Big Bint will be OK and we can all settle down and get back to normal (though I've forgotten what normal is, and whether it's desirable). At least we can get back to work. Thank you for those who sent good wishes or gave more tangible help during the summer crisis - I'll book you a ringside seat for the autumn crisis, as crises seem to be coming with seasonal regularity.
Now back to the publishing contract. It's a clause from la-la land (which I think has been rebranded TellyTubbyTopia and have an R in a circle after it) - this is never going to happen!
20. Overpayment [yes, that's right]
Any overpayments made by the Publishers to the Author in respect of the Work or any other of the Author's works may be deducted from any sums due to the Author from the Publishers in respect of the Work or any other Work, but an unearned advance shall not be deemed an overpayment unless the Author has failed to deliver the complete Work.
This means that if the publishers send you too much money [pause for hollow laugh] they can claim it back from other money they owe you either for the same book or for another book. It's not worth stopping to say 'what if I've spent it? it was your error' because it's so unlikelyt that a publisher will ever pay you too much. And as long as the clause does, like this, say they will claw it back from other money they owe you, it's not going to be too hard. You don't want them demanding you send it back directly, though. If you have to argue about this last issue, you can point out that it is entirely within their power not to overpay you.
Perhaps the most likely type of overpayment is wrongly calculated royalties. Unless it's an error in the maths (unlikely, as they are calculated by computer), that's very hard for you to check. But I've never been overpaid by a publisher and I've had around 130 publishing contracts. Slightly fewer, I suppose, as series are often covered by one contract. But most contracts cover many payments, so that evens out.
It is important that they do not count a too-optimistic advance to be an overpayment. The advance is based on their judgment of likely sales. You shouldn't have to pay for their poor judgment. In fact, there are authors - and agents - who seem to think the whole point of publishing is to trick publishers into making bad judgments about the likely sale of a book and so securing an implausibly large advance and laughing all the way to the bank. It's nice to have a large advance, but it backfires if the publisher realises you are not even as good as sliced bread and makes it your last advance.