Verizon Wireless on Monday blamed a software glitch for allowing customers to upgrade to a subsidized handset while keeping their grandfathered unlimited data plans, despite a year-old policy change prohibiting that arrangement.

Verizon last year killed off its unlimited data offering, instead opting for a tiered pricing model and family data plans that offer customers a bucket of data. Customers already on the plan could keep it unless they upgraded to a new phone and chose the subsidized price, at which time they would be forced to give up their unlimited data for a capped plan.

However, a “software issue” at Verizon’s Web site this past weekend allowed customers to upgrade to a carrier-subsidized phone while retaining their unlimited data plans. The carrier said in a statement to Droid-Life that “a number of customers” took advantage of the glitch, which it said was fixed as of today.

But customers who managed to get this deal will be allowed to keep it, Verizon said.