A t-appalling mistake to make – having to tap in and tap right back out (Picture: Getty)

It’s very possible you’ve done this before – you’ve tapped into the London underground station and just as you’re about to huff and puff your way to the platform, you realise you forgot something.

Work laptop (another argument for working from home). Birthday present. Child (okay, hopefully not your child). But rule of threes and all.

If it’s something you really can’t spend your day without, you have to circle back and tap back out.

Other than risking embarrassment – a price far higher than any monetary cost – will you actually be charged for a journey?

We all know how the tube zone fares work, but here’s what happens if you tap in and out at the same station within certain time frames…

Will you be charged for tapping in and out of the same tube station?

So, Transport for London (TfL) have a whole page dedicated to this – an encouraging sign that you’re not alone in having to tap back out after forgetting something.

And they reveal that you actually will be charged… but there are caveats.

Got to the platform and realised you left the TV on? Or your windows open? (Picture: Getty)

It all comes down to trying to clamp down on fare evasion. For example, in the worst case scenario, only tapping in and out of the same station could, to a more cynical eye, imply that you’ve sojourned all over London but not bothered to tap out at the other stations, jumping over or sneaking under the gates instead.

If you tap out between 0 and two minutes after tapping in, you’ll be charged a full fare.

This means you’ll be charged as if you’ve taken the tube the maximum journey. A single maximum fare can be up to £9.40 between Zones 1 and 9.

Now, if this seems unfair, the good news is, if you actually then come back and tap back in up to 45 minutes after tapping out, you’ll be refunded  on that maximum fare and only pay what you actually travel.

This seems to indicate that they know some people might genuinely have forgot something or left the iron on. Returning to finish your journey within the hour implies you’re not simply dodging fares.

How much you’re charged if you tap back out without making a journey changes the longer you wait.

If you tap back out:

Between 2 and 30 minutes, you will pay the minimum pay as you go fare from that station

More than 30 minutes later – TfL assume two separate journeys have been made and both will be incomplete. You’ll be charged two maximum fares.

Can you get a refund if you’re overcharged?

You can get a refund if you have been charged for touching in and out at the same station without making a journey.

TfL advise that, before making a claim, they aim to automatically refund you for a same station exit if you have not had one in the last seven days.

However, if you haven’t received an automatic refund after 48 hours, you can submit a claim for a refund on your contactless and Oyster account online.

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Will forgetfulness cost you?