The UK's HMRC is alerting citizens about potential unexpected tax refunds exceeding £3,000, with millions of households facing tax bills due to reaching a limit. Martin Lewis, a prominent financial expert, has issued multiple warnings regarding changes affecting savings accounts and credit cards. He highlighted that six million people may be subject to a 7.75% tax charge as an important deadline approaches, with HMRC sending out letters to inform those owed cash refunds. Additionally, Lewis cautioned that four million individuals have overpaid taxes and should reclaim their funds. He also noted that many customers, particularly those with Barclays and HSBC, need to be aware of under-the-radar changes that could impact their finances. Furthermore, Lewis emphasized that millions are missing out on free childcare benefits, urging immediate action to claim available funds.