Why can a transit gate charge the wrong card?
Because more than one credential answered and the reader took one of them. A phone held against a wallet, two cards in the same pocket, or a travel card beside a bank card all produce it, and which one wins is not something you control at the gate.
What actually happens
A reader powers everything in its field and then has to choose. The procedure that sorts several answers into one conversation is anticollision, which the anticollision page describes, and it selects a single card rather than refusing.
Whether it refuses instead depends on the reader. A gate that detects several cards and stops is behaving well, which the two tags page explains, and one that quietly picks is behaving badly for the traveller and normally for the protocol.
Either way, nothing about the wrong card was defective. The field reached two credentials and the barrier had a fraction of a second to act.
The phone makes it worse
A phone in a case with cards behind it is exactly the geometry that produces this, and the wallet page covers what a stack of cards does to a field.
The phone's own credential is not the only thing present: a travel card, a building badge or a bank card in the same case is powered by the same reader, and the badge is often the one that answers first because it sits closest to the glass.
A phone can also be the innocent party. A wallet of two bank cards produces the same outcome at a gate with no phone involved at all.
How to avoid it
Present one thing. Take the phone out of the case, or the card out of the wallet, which is unglamorous and works every time.
Keep travel credentials and bank credentials apart physically, because the ones you want to keep separate are the ones most likely to be adjacent in a pocket.
If a case must be used, know which side reads and present that side, since the antenna position decides what the gate sees first, as the antenna page shows.
And use the same device for entry and exit, since a journey opened by one credential and closed by another is charged as two incomplete journeys, which costs more than either.
What to do afterwards
A wrong charge at a gate is a transaction like any other: it appears on the account that answered, and the operator's own process is what corrects it.
Keep the pattern in mind when it repeats. A gate that charges the badge every morning is a geometry problem you can fix, not an account problem to report twice a week.
And do not expect a phone setting to solve it. What the gate sees is decided in the field around the reader, before any software has an opinion.
Frequently asked questions
Why did the gate charge my other card?
Both were in the reader field and it selected one. Anticollision picks a single answer rather than refusing, unless the gate is built to detect the clash.
Does a phone case cause this?
It is the commonest cause. A card behind a phone sits closer to the reader than the phone credential does.
Can I stop it in the wallet settings?
No. The choice happens in the field before software is involved. Present one credential at a time.
What if entry and exit used different cards?
The system sees two incomplete journeys and usually charges both at the maximum. Use the same device for the whole journey.