lucy24 - 10:18 pm on Nov 5, 2013 (gmt 0)
Is the first visit always from yahoo or was that just a random example? If it is always the same referer, you could hypothesize that the first visit is the search engine's humanoid UA doing some pre-loading business. This has been discussed in other contexts (chrome + first-google-result is what comes to mind).
Is everything a page file? I've seen the user-agent switch occasionally myself-- but now that I'm actively looking for it, all examples involve people apparently returning for image files.* Or people on mobiles coming back in a different browser. MSIE 7 vs. 10 is weird, though.
Now, about that explicit "index.html"
:: tweet tweet, chirp chirp ::
* Ones that they are perfectly welcome to keep on their HD for reference. I am not an artist ;) The most recent specimens are either keyboard layouts or illustrations from public-domain books.