It happened multiple times.
I was browsing the main page of SO and I clicked on a question, and while the request was being sent, my eyes glanced over the SE network question-type ad on the right. I had time to read the question and it was relevant to my interest (although unrelated to programming), but the page concerning the question I requested loaded.
I quickly pressed back to re-read the question and be able to follow the link, but once the page reloaded, it was displaying another question that was no longer relevant.
I can't believe I'm asking this concerning advertising, but I'd like to be able to press back and still have the same ad. Maybe use a 10 to 15 seconds cache on ads.