You sometimes see this in the WWW when a page with frames links to one without. When you click the link, the new page can be loaded into the frame the old page was in, leaving old content in other surrounding frames untouched. (Generally this is due to badly written HTML so it is pretty rare).
It can happen on some wikis through the PageRedirect feature, although only the title of the original page is kept, which limits the danger.
It is at best irritating. It can also imply that the included page sponsors or condones the including one, which may be misleading or deceitful. Changing the context of a page can change its meaning. For example, a comment on "Why I like Microsoft" may show up under "Why DaveHarris is a pratt".
This is pushing ridiculousness. Could you imagine forcing every web author to litter her pages with landmines, barbed wire, and hidden ambushes, just to ensure that her page is viewed in the "right" way? Of course, this isn't ridiculous due to the interconnected nature of the Internet. Perhaps this is why TedNelson gave up and invented TransCopyrights.
Should this page cover "BandwidthTheft" of images, or is that/should that be a seperate issue?