There seems to be a problem consistently drawing the interior of an element using -webkit-border-image. Just resize the window and watch the blue lines. If the page contained enough of these elements to force a vertical scrollbar, the lines might change positions (slightly) when scrolled out of and back into view.
Test case is gone, so nothing can be done here.
I've reenabled that webserver, though it looks like the png used for the border image is missing. I'll try to find or recreate it.
I couldn't find the original pic, but I substituted a similar one and tweaked the css so that it demonstrates the problem. The blue lines (in the border-image) can be seen to "wiggle" vertically if the browser window is resized vertically. if I make the browser window small enough to force a scrollbar and then scroll up or down, an individual line will sometimes disappear or be double-thick. I can't reliably reproduce the disappearing/doubling lines, but I've seen it multiple times. I'm currently using webkit-svn-r95145 and OS X 10.6.8.
Thanks for restoring the test case! I can reproduce this issue with Safari 5.1 on Snow Leopard, but not on Lion. WK1 and WK2 modes work identically. Not sure if there is more to investigate here - could be a problem in system frameworks below WebKit.
This doesn't repro on Lion even before all my recent changes, so I'm assuming it's a bug in CG on Snow Leopard.
I can't reproduce this at all in Safari 6.0.2 on Mountain Lion. Since this was previously reported as unreproducable in Lion, I'll guess this really is fixed. :)