It pays to read the fine print on your phone plan:

A Canadian man has been shocked to receive a mobile phone bill for nearly $85,000 (£41,000). Piotr Staniaszek thought he could use his new phone as a modem for his computer under his $10 unlimited mobile browser plan from Bell Mobility.

Of course, checking with staff ought to suffice, but as Ed Foster notes in a related story, even that's not always enough.