Found a bug in

template/tt2/diff_button.html
[download]

The correct code (obtained from here) is this

 _template/tt2/diff_button.html__
<!-- BEGIN diff_button.html -->
<a href="[% script_name %]?action=diff&page_name=[% page_uri %]&revisi
+on_id=[% revision_id %]" accesskey="r" title="Differences">[% INCLUDE
+ diff_button_icon.html %]</a>
<!-- END diff_button.html -->
[download]

I'll drop an email to the owner on this.

Happy weekend, all.