I heard about them back in olden times but never saw one in practice.
You almost certainly see them all the time but don't know it ;) Next time you run into an auto-generated directory-index page, check the source. It will probably be html 3.something. (MAMP uses 3.2 Final. I just checked. Final, wow, that's decisive.)
:: detour to image ::
Mm, interesting. Does it continue to do it if you fiddle with the browser window? Not the code, the physical window: resizing or simply moving it around a bit. The Safari version does seem a bit skewed, as if the curves are 4px one way and 3 px the other.
It helps to put everything into a distinctive color-- including the kinds of horrible contrasts you would never use on a real www page-- to make it easier to see what's overwriting what. I notice that the Chrome version is also a few pixels to the right of the Safari version, though the object sizes and spacing are otherwise exactly the same. Is there a reason for this? If not, it's another thing to deal with: I make it an extra five px offset.