iOS 8.4 Update - data usage I have started the update to 8.4 last night, while my phone was charging and connected to the Wi-Fi. This morning it was still working on something (the black off screen with the loading bar and apple), and then it asked for my passwords, my touch id, my iTunes account etc. 

I entered the passwords etc., it got accepted and then I left my house (therefore my Wi-Fi). 10 minutes later I receive notifications that I've used 900MB of data and I owe £19 for overspending. 

How is this possible? Isn't it supposed to only update when connected to Wi-Fi or with my specific permission? I have no idea how to spend 900MB in 10 minutes. Please give me an official message from Apple saying that it will not update unless on Wi-Fi, or specifically approved.
I do not know if I had this problem because I have unlimited data, but my friend did. Notably, she also has a different carrier (Verizon) with limited data and when she noticed the high data usage she contacted Apple. They acknowledged that with some carriers this is a bug in the newest version of the iOS. The fix they had her implement, until the next version of the iOS comes out, was to revert to the prior version. Since the update had completed, she was forced to do this through a full reset. Since the phone came with an earlier iOS version, this is what it reverted back to and the issue was solved. Again, I am only reporting this as an observer and have not fixed/experienced personally, but thought I would offer this in the event it helps.