One of my clients had some very interesting Internet traffic statistics last week. We came in Thursday morning and found that overnight we had downloaded over 700 GB of data from our ISP (UQ SchoolsNet).
When we looked through our proxy server logs we found that 538 GB of the total came from a single PC attempting to download a single URL for Adobe Acrobat Reader 9.2 updates. Fortunately, we're on an Internet plan which is capped rather than charged for excess traffic, and more fortunately still, our ISP hosts an Akamai mirror, which is where the URL for the updates resolved to. So, no harm done.
What this reinforced to me was that allowing direct access to the Internet by PCs is rather irresponsible, both from a bandwidth utilization perspective and a cost perspective. (And that doesn't even take into account what legal ramifications there might have been if it had been a BitTorrent client rather than a misconfigured/buggy software update client.)