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.