http://www.engadget.com/2010/02/04/panasonic-cops-to-rising-black-levels-in-its-plasma-hdtvs-but-q/

A feature that made the black level brighter over time on plasma TVs was miscoded and does it faster than it is supposed to, causing black level problems with older plasma TVs.

Now the question is, is it really necessary for them to do this, or is this a planned obsolescence feature to make new TVs appear more attractive when they come out?