Want to stream some music from your Xbox Music cloud collection to your Windows Phone 8 device, over WiFi? Not so fast. You might end up eating through your data plan, like I just did.
The fundamental problem here is that Xbox Music is unable to switch to a different connection once it starts streaming – so if you start streaming via mobile data and then connect to WiFi later, it’ll continue to use the mobile data connection. But I’m at home, so my phone should be connected to my WiFi network already, right?
Wrong. One langstanding bug/annoyance in Windows Phone is that it turns off WiFi when the screen is locked. Which means that if you start streaming Xbox Music during those several seconds that it takes the phone to reconnect to the WiFi network  – right after unlocking the screen – it will use the mobile data connection. And ignore the WiFi connection.
Now, this might sound like an edge case, and it may well be; all I know is that I already encountered this edge case twice: yesterday, just after I received my HTC 8X, and today. Thanks to this behavior, I’ve already used up my monthly data allotment of 200MB, which usually lasts me for at least half of the month.
Hopefully Microsoft will fix this issue (and add an option to keep WiFi alive when the screen is locked), but in the meantime, always make sure that your WP8 phone is already connected to a WiFi network before hitting that “play” button to stream music from the cloud.