Taxwise, an old friend of the family was a scrupulously exacting fellow, in all ways, and as a sole-trader/handyman had to do all his tax/accounting to present to the (UK) taxman. Every year he claimed so-and-so for reasonable work expenses (clothing, materials/etc) and every year the paperwork allowed him maybe 80% of that. Which he took as normal.
I forget how, perhaps he made a query about something that attracted the attention of the tax-enforces, or maybe it was a random spot-check, but suddenly one year he was faced with a thorough audit of the "show me the receipts for everything, show me the invoices for everything, show me your entire set of books..." kind.
At the conclusion of which, they conclusively proved that... he was owed that 20% back in tax refunds, over the last however-many-years! Unlike others, he had not overclaimed, but he'd still somehow been penalised by the system meant to normalisre 'standard' overclaiming.
(Anecdotal, some of this, as a family tale, but I could indeed believe that he was so straightforward and straight-laced, though I've got no idea how/if "pre-emptive anti-fraud" actions happen at the tax-office end. For all I know, it was a rogue beaurocrat somehow skimming funds, or the true story was more like he hadn't been told that certain expenses were includable. Can't even go andcheck with anyone, as nobody who really would know is around any more.)
I have always rather trusted in my PAYE tax-code to do the heavy-lifting for me (with a few rare exceptions, it covered my actual income from employment/blah-de-blah) and I don't believe there's much I could have shaved back from the system by discovering some sort of expenses loophole. Which maybe is what fuels the economy, such passive belief in the system, or maybe it just saves me time/energy/money in getting a better outcome but at some cost (e.g. hiring an IFA, registering as a charity or getting a Cayman Islands holding account - any or all as applicable) such that anybody but me ultimately wins out.