Since we're talking about obscure hacks, I've seen a website putting ads in an <iframe>, whose URL was the same as the parent (top) URL, and there was even no HTTP request being done for an iframe document (probably due to the URL being already cached by the browser) - so it was not possible to block it via a blocking proxy.

I only managed to block it with the adblock's element hiding query like `domain.com##iframe` or through a userscript.