In Akismet 2.3.0 there are two non-updated functions in the akismet.php file, which have been deprecated since WP 2.8
If running WP with WP_DEBUG = true, then the main WP functions file catches them and displays page top warnings in admin, and http 500 errors public side.
The two deprecated functions are -
register_sidebar_widget (3 instances)
register_widget_control (1 instance)
all four instances need wp_ prepended to the function name
This errors may not manifest if you don't have a widget active and only use template tags in your sidebar.
( ... am thinking we need a trac system for bug reporting WordPress supported plugins).