This happened to me about a month ago and I thought it was just an isolated mistake for Verizon. I added the double your data in online chat when they first offered it. The next day I noticed my cap had changed from not 2 GB to 4 GB but from 2 GB to Unlimited! However, the online rep must have accidentally added mobile hotspot as well since my bill was $60 more my next month. I called and angrily asked them to take off the mobile hotspot and credit me, which they did. The Unlimited data remains though... And I won't call and ask why
Even my bill changed from $30 for data to $29.99 to data like the grandfathered unlimited data people.