Originally Posted by jdsva

This is true, but I'd be willing to bet it could still cause issues for some transit systems where they aggregate your charges for the day. Just because they have access to more detailed information through the chip doesn't mean some developer didn't use the card number as the primary key in the database.

Previously it was mentioned about hitting fare caps, but the place where I think it would be particularly bad is those transit systems that track your in & out places. Once card X has entered at a location, they can't enter again without exiting (so it knows what fare to charge). So if person A with card number X enters and their significant other with AU card X tries to enter behind them, it might not work.

Again, this assumes that someone used the card number as the key in the database, but I'm also certain there is a system out there where this at least WAS a problem at some point.

Originally Posted by rhwbullhead (Post # 21)

Here are some other ones I've had before that definitely have unique numbers for AU:

Comenity Caesars Rewards
Citi Retail Shop Your Way

Originally Posted by MASTERNC (Post # 28)

My wife is an AU on my Citi Costco card and her account number is different than mine.

Originally Posted by Wyfind

I ran into the in-and-out problem on the Singapore MRT.

My mom uses her Visa card (same account number as mine, as she's an authorized user on my account) to tap in before me, which works fine as expected. I tap in with my card (same card number), and I can't get past the gates.

Originally Posted by rhwbullhead

Here are some other ones I've had before that definitely have unique numbers for AU:

Comenity Caesars Rewards
Citi Retail Shop Your Way
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Originally Posted by MASTERNC

My wife is an AU on my Citi Costco card and her account number is different than mine.

Originally Posted by jdsva

I think Costco mandates this due to membership / picture on card requirements.