If, and apparently only if, you have a beforeload handler installed, subsequent loads of already loaded images gets bogus dimensions for the element in the onload handler.
Read source on the URL above for a good reproduction.
Believe it or not, commenting out the beforeload handler makes this go away.
This is a minimisation of what users were seeing in chrome bug 72484: http://code.google.com/p/chromium/issues/detail?id=72484
See also: bug 45586.
And I'd be shocked if https://bugs.webkit.org/show_bug.cgi?id=52581 wasn't closely related, too.
A fix will make a vocal subset of AdBlock users very happy :)
I think this is actually a dupe of bug 45586. Please correct me if I'm wrong.
*** This bug has been marked as a duplicate of bug 45586 ***