The cracks in Apple's new contactless system (or perhaps, more specifically, the lifetime of its devices' batteries) are starting to show.
As part of a detailed set of advice for commuters using their iPhones and Apple Watches to effortlessly waft through ticket barriers on the Tube, TfL has warned of the very real possibility that their devices could run out of battery on the journey.
“Check that you have enough battery on your iPhone or Apple Watch to complete your journey,” it says. “If you don't and it runs out of battery in the middle of a rail journey, you will not be able to touch out at the end and could be charged a maximum fare.”
Likewise, if an inspector asks you to touch your iPhone or Apple Watch on their reader, “it will not be able to be read and you could be liable for a penalty fare”.
Read more: Why investors are wrong to punish Apple
Crumbs. Given how much TfL is already making out of commuters forgetting to tap out (£8m in just a few months according to the Evening Standard) this could end up being quite a costly way of travelling.
That's not all.
If you use Apple Pay on more than one device, for instance when the same payment card is linked to both an iPhone and an Apple Watch, you could be charged for an incomplete journey, and you'll miss out on daily or weekly caps.
And overseas travellers using Apple Pay could be charged currency fees (that's if it works at all).