Forwarding this from a Fedora bug report:
http://bugzilla.redhat.com/show_bug.cgi?id=429929

Occasionally I receive e-mails with words, such as URLs, which are longer than
the usual 80 or so characters.  (Often these messages are also very long
vertically.)  When I view such a message in Evolution, the viewer scrolls right.
 If I scroll left while the message is still rendering, it scrolls right again.

I would expect to see the leftmost part of such messages at the end of the
rendering process, since English reads left-to-right.  Scrolling right should
only be done manually.

evolution-2.12.3-1.fc8


(Milan's comment)

I cannot reproduce this consistently, furthermore, it did to me only once. When
I started to investigate it more, it started to act as expected. I suspect that
some part of the code changed horizontal scroll bar position beyond the
document's limit, so GtkHtml thought you have scrolled to the right already, but
too much to the right, so it decided to correct it to maximal right for the
rendered document. Which is the right thing in case you did move to the right by
hand, not by program itself.

As I said above, I cannot reproduce it again, maybe some race condition or
something, in GtkHtml probably (I suspect gtk_html_private_calc_scrollbars).