Many public transit systems now use RFID cards for fare payments. Sometimes, multiple cities purchase systems from the same vendor, or use the same design, such that if you put cards for both cities on the reader, it would try to look up an account associated with the other city's card number, and either not find an account (& thus report the card is invalid) or in a rare case, charge some other random person's account.
For the purpose of this question, payment card/ID systems "interfere" if an attempt to read one credential on another system produces an error signal or unexpected behavior, different from what would happen if someone attempted to use blank paper or cardboard as a credential. For good answers, they do not have to both be transit systems (e.g. if some credit card system or large university access ID card or hotel chain key cards interfered, that would be good to know).
As a result of knowing the answer, a traveler with more than one city from a given interference set on his/her itinerary would know to keep the cards separate from one another and only hold up the correct city's card to the reader, instead of e.g. holding up a whole wallet or purse that contains the intended card as well as interfering ones.