T-Mobile is flouting one of the biggest taboos of the mobile industry: the monthly data allotment. T-Mobile CEO John Legere on Tuesday announced a new program called Data Stash, which allows you to rollover unused data each month into a kind of 4G piggy bank and use it for up to a year if you exceed your normal data plan.

Typically you buy a mobile data plan that comes with a set number of gigabytes or megabytes each month, but whatever you have left over at the end of the billing cycle disappears into the ether. That’s left consumers with two equally unattractive propositions, Legere told Yahoo’s David Pogue in a webcast interview. Customers either lowball their monthly data usage and wind up paying overage fees or they overestimate their data use and often wind up with leftover gigabytes each month, Legere said.

Data Stash will let customers bank…