Since some time, the SE network has "Hot Network Questions" in a sidebar. Interesting as they are, I click on one of them. It occurs quite frequently that I see another interesting question after having navigated away. When pressing the Back button to see the previous page, I would expect the same list, but the list of questions have changed.
NOTE: this is not a duplicate of the question Ease up on the Hot Network Question Randomness which is about update frequency. Rather, it is about the list being updated even if just navigating through history.
After an hour of trying to make a test case that replicates the HTTP conversation, I have finally found that the issue is related to Websockets. When these are enabled on a page to which you return later, the browser cache is refreshed. This probably affects more than just Hot Network Questions, it effectively prevents caching from working at all!
Browser: Firefox 28 on Arch Linux x86_64.