It's the financial nightmares you never thought would happen that really hurt. So here’s how to fight back against the top 10 hidden bill perils.

1. “My six-year-old spent £3,200 on an iPhone game”

Letting youngsters sit on your knee while you use your smartphone or tablet means they're likely to know your password, and that can be expensive.

Recently, England rugby player Sam Vesty got smacked with a £3,200 bill after his six and eight-year-olds bought their virtual farm animals a virtual mountain of food, with real cash, at £70 a pop over three hours. This is just one of countless examples I’ve heard of. It's disgusting that a kids' game allows this, but it happens, so protect yourself.

If you’re going to let the kids use your tech, there are tips to follow. Protect your password – your kids may know it without you knowing, so change regularly. Plus ensure your phone’s “in app purchases” setting is restricted, so it needs a password. Most phones let you do this. Also speak to your network about financial and parental controls.

An alternative with iPhones is to delink your credit/debit card from your account and buy vouchers instead. Then it’ll never go over the top. And, finally, if all goes wrong and you’ve been stung by a massive charge due to the kids, do call up and explain. Often they’ll wipe it on a ‘one strike and you’re out’ policy – as they did for the rugby-playing dad.

2. Avoid paying £150 a month for busting your overdraft

Break your overdraft limit by even £1 and you can face charges of up to £5 a day or, at Clydesdale, up to £35 a transaction. It's very easy to get caught out, but extremely costly, so ensure you stay in the black.

If you have been caught with a hefty amount of bank charges, many of you will remember a few years ago I was urging people to reclaim them. Some got thousands bank. While a decision in the Supreme Court put an end to that, rumours of the death of bank charges reclaiming have been greatly exaggerated.

If you’ve had charges and they’ve put you in financial hardship, you may still be able to reclaim. Full, step-by-step help at moneysavingexpert.com/bankcharges.

3. Beware traps offering £15 off your next purchase with an online discount

Shockingly, even sites like The Trainline and Ticketmaster have reportedly made an extra 30 pieces of silver by allowing membership clubs like Shopper Discounts to push these offers once you've bought stuff.

Many have been caught out, as MoneySavingExpert.com forum user Sweetie27 wrote: "Bought a train ticket and must have clicked a link, as for two years Shopper Discounts has been taking £10/mth from my account, now totalling £200. I did not know anything about this and am totally gutted."

Communications about these have marginally improved since then, but be very careful – my view is these aren’t worth signing up to.

4. Beware debit card stealth charges when abroad

Santander, NatWest, RBS, Halifax, Lloyds all add up to £1.50 every time you use them to spend on them overseas. AVOID!

Instead, get a specialist overseas credit card – Halifax Clarity, Post Office, Saga (over 50s) and for Nationwide account holders ONLY, its Select card. These have no spending charge, low ATM fees and crucially they don’t ‘load’ the exchange rate, meaning near perfect rates worldwide.

Yet the golden rule is to set up a direct debit to repay these cards IN FULL each month to avoid interest. Full help and full card-by-card breakdown at moneysavingexpert.com/travelcards

5. Watch TV online? Don't pay hidden £5 per film

Web players now pump out programmes with image quality rivalling Sky and Virgin. Yet this hoovers up data, and many broadband packages have data limits. You can be charged £5 per 5GB (about two HD films) if you're over your limit. Consider an unlimited package, normally only a couple of quid a month more expensive.

6."I got a £3,000 holiday mobile data bill."

Many smartphone apps routinely check for updates and downloads in the background, and this can lead to monster fees abroad. Watching TV, videos or streaming music is worse. Prices aren't regulated outside Europe, so turn 3G off. One of my users got stung with a £3,000 bill just for a few emails and maps on a trip to India and the USA, and leaving their data roaming on when not using it.

7. Don't `nearly’ repay cards in full - you'll get an unexpected slap

Repay credit cards in full and you usually don't pay any interest. Yet if you owe £5,000 and repay £4,999, many cards will still charge you that month's interest on the whole £5,000.

8. Beware mobile voicemail, which can be 35p a minute

Voicemail isn't always free. Many firms charge a hideous 35p a minute if you call it when you've used up your inclusive minutes (or from abroad). Keep track of your spending, or turn it off while on holiday.

9. Fixed your energy price? Watch for pricey 'go-to' tariffs

Once a fixed energy tariff ends, you slide onto providers' uncompetitive standard tariffs - up to £260/year more than the cheapest. So diarise when it ends and switch again or join my new moneysavingexpert.com/CheapEnergyClub which does just that for you.

10. Have debts where you bank or save? Beware

If you've credit cards, loans or mortgages at the SAME bank where you save, beware. Banks can legally "set off" or use your cash to repay your debts without asking you. They tend to do it if you're struggling to repay.

This has cancelled Christmases and left many in misery. One proud dad told me he paid £12,000 to his daughter's account for her big day. Two days later, the bank used £6,000 of it to pay off her credit card debt even though there was a repayment plan in place. It's legal.

If you’re at risk, the golden rule is to simply separate them, and use different financial institutions to save and to borrow.