Cellular data on wifi WTF?

Today I received an increasingly hysterical series of text messages from Telus warning me that I had exceeded my monthly data allowance, culminating in getting cut-off early this afternoon for going $50 over my allowance.

Logging into Telus's website revealed that my iPhone 5 had succeeded in burning through 3.5 GB of cellular data on App Store downloads while:

a) being connected to wifi;

b) without showing a single app being updated in the last 6 days;

c) while Use Cellular Data was disabled in iTunes & App Store settings (it's not clear how this setting is distinct from the “Use Cellular Data for…App Store” switch in Cellular settings (which had been enabled until about 5 minutes ago)).