Update: Coin already has this covered. They’ve responded to the numerous people asking this question.
One question we’ve been hearing is “What if a waiter/waitress accidentally — or worse, INTENTIONALLY — changes the card you want to charge your meal to?”
Here’s where Coin has you covered: The mobile app will allow you to configure an auto-lock feature that will disable the Coin button to toggle based on proximity; when the waiter walks away he or she will not be able to toggle the card selected.
I’ve pre-purchase Coin because for years I have said that we should have a single magnetic swipe card for all purposes. I’ve gone so far as to suggest we be issued an id card at birth that becomes our permanent id and holds all our credit cards and so forth. Why have multiple cards when one with a programmable magnetic strip would suffice?
Coin holds 8 swipe cards at once. There is a button on the card which allows you to select the card you want to use. Perhaps card one is your primary credit card, card two is your business credit card, card three is your door entry key for your office, card four is a loyalty card, card five is that rebate card you were sent instead of receiving a check or cash, card six is your department store card, etc. Pressing the button cycles through these. The recurring question being asked is "What if I hand it to the waiter and he accidentally selects a different card?" Coin answers this question in their FAQ:
Q. Can someone accidentally change which card is selected on my Coin?
A. We’ve designed the button to toggle cards in a way that makes it difficult to trigger a “press” unintentionally. Dropping a Coin, holding a Coin, sitting on a Coin, or putting the Coin in a check presenter at a restaurant will not inadvertently toggle the card that is selected.
[Source, onlycoin.com, FAQ]
To me, the answer is inadequate. The user interface could be altered ever so slightly to address this concern.
The solution: For those who want to lock it in, I should be able to program a lock sequence into the button. For instance, make switching cards a short press (that way I can rapidly cycle through the 8 loaded cards). I go to my Coin app on the phone and define a lock sequence of a 3 second press, followed by two tabs, a long press and a final short tap. Now the card is locked on the chosen card. Unlocking works the same way. Because I can define the sequence of short and long presses, I effectively have a pin code for locking and unlocking the card.
Perhaps Coin will consider this in the future.
For those asking, what is Coin? Please read One Card to Rule Them All.
Update: Lauren Puff suggests the use of finger print technology like that of the iPhone 5s. While I think that would be cool, it would require a hardware change to the Coin which would probably add to the manufacturing cost and may not work with the form factor. The lock sequence I propose would use the existing hardware and would be some minimalist programming changes to the device itself and the iOS/Android apps.