My cellular carrier just alerted me that we've exceeded our data usage. They show that I used more than 500MB at once a few days ago at a time when I was at home in bed, and the phone in range of my WiFi. I assume the phone downloaded an update automatically, but there was no reason for it to use cellular data when WiFi was available.
The device is a Nexus 5 and it recently updated its OS. I don't have it nearby or I'd check the version.
Can I set an Android phone to always use WiFi, for all apps, if its available?