An error occurred during the processing of . Unknown server tag 'asp:ScriptManager'.

So I have changed the default.master for testing some thing and have forgotten to revert it back.
This is what I ended up with when I attempted to open Central Administration default.aspx page.
"An error occurred during the processing of . Unknown server tag 'asp:ScriptManager'"

So went back to the 12 hive Global folder and reverted the default.master to its original state.
Central Administration's default.aspx page uses default.master.