This is a growing problem as more ISPs see caching as a way of decreasing net traffic. Read the note on caching web servers even if you don't think you are using one. We also suggest you use lynx when testing.
Many people route web requests through a cache.
This is essentially a web server that has no content of its own. It sits in the middle between users and real web servers. The cache grabs requests for web pages, fetches the page and makes a local copy. This can really cut down on net traffic when many users request the same page. This is a great idea most of the time, but fails when the cache is buggy.