BUG #7942: Timestamp "19991231 240000" should not be out of range

The following bug has been logged on the website:

Bug reference: 7942
Logged by: Georg Kindermann
Email address: georg(dot)kindermann(at)gmx(dot)at
PostgreSQL version: 9.1.8
Operating system: Debian Wheezy
Description:

select cast('19991231 240000' as timestamp);
--Produces >ERROR: timestamp out of range<

select cast('20000101 000000' as timestamp);
--Works

--For other years both styles work e.g
select cast('19981231 240000' as timestamp);
select cast('19990101 000000' as timestamp);

--1999 to 2000 is no Leap second. Is there something other special? Thanks!