Consumers have complained that Panasonic's HD televisions have displayed changed in their black levels after extended viewing.  An AOL article said,

"In order to achieve the optimal picture performance throughout the life of the set, Panasonic Viera plasma HDTVs incorporate an automatic control which adjusts an internal driving voltage at predetermined intervals of operational hours. As a result of this automatic voltage adjustment, background brightness will increase from its initial value. The newest Viera plasma HDTVs incorporate an improved automatic control which applies the voltage adjustments in smaller increments. This results in a more gradual change in the Black Level over time. especially considering many buyers purchased their televisions specifically for those deep black levels, you can see why a TV suddenly going Sammy Sosa overnight would be upsetting. One of the reigning theories in the thread indicated by poster & calibrator D-Nice has been that this is by design, but a flaw in the settings caused the large jumps (around double the brightness, as measured by several owners light meters) instead of a much more subtle change. So what now for owners or potential buyers? Without more details about what is going on and whether or not anything can be done about it, like CNET's David Katzmeier, it's hard to see how we can continue to recommend these HDTVs for purchase without knowing what they will do months or years down the line. The ball is in Panasonic's court now, a speedy response could do a lot to assuage the concerns of current and potential owners."