Users have complained of a very serious bug in Windows Phone 7, in which the phone uses its 3G connection even when connected to Wi-Fi.
Julie G, writing to the SuperSite for Windows mailbag, says:
I got my Windows Phone 7 the day it came out, and as of yesterday I completed my first full month of service. With this phone I do not stream, I hardly surf the internet, I mostly have the location services turned off. I do have Facebook connected with Windows Live so I do get the updates on the people hub and pictures hub and I do play Bejeweled quite a lot as I am addicted. I do have my Yahoo mail and Outlook mail synced to the phone. Beyond those few things, I do not do much else with the phone.
On December 22nd I received an email from AT&T saying that I was close to my 2GB data limit which truly shocked me as I feel I do not use data that much. I went and looked at my AT&T account online and noticed that my phone was sending huge chunks of data seemingly in patterns. For instance on November 21-24 it sent between 30 and 50 MB of data at 10:41pm each day and Dec 1-4 it sent between 30 and 50 MB of data at 9:41am each day. On December 23rd I turned on airplane mode so my phone could no longer send data. I turned airplane mode off briefly on December 23rd and the phone sent 400 MB of data. I called AT&T yesterday, December 28th, but they said that there was nothing that they can do to figure out what was happening on my phone.