LCDs...

[Tab]

What causes "dead pixels" and backlight bleed through... What went wrong in the manufacturing process?

 

[Calin]

It could be wrong or dead electric connections to activate the pixels. Or it could be also a problem in the polarising film.
A LCD screen has a backlight, and that light is polarised thru a filter. Another substrate, parallel, is the "programable" polarizer. The "programmable" polarizer is capable to either polarize the light in a direction rotated 90* or to let it pass (or polarize in the same direction as the other filter). When the direction of polarisation is rotated 90*, the screen blacketh ( ). If not, the light can go thru, and a lighted spot appears.
The "programmable" polarizer is controlled with voltages (electric fields, like in condensers). The presence of the field changes the polarisation plane, compared to the absence of the electric field. Those "cells" (which are sub-pixels on a color screen) keep their information thanks to some "static RAM" cells, so no refresh is needed.
Well, when one or more of the transistors are defective, the (sub)pixel can be either dead (black) or stuck (white).

 

[cirthix]

good explanation cailin. just to add: the amount of light that comes through the lcd itself is modified by changing the opacity of the liquid cyrstal, then the light passes through a color filter (think of collored cellophane for simplicity), giving us the colored subpixels

 

[itachi]

not exactly.. what you're saying is that the opacity of the lcd is modified by changing the opacity of the liquid crystal.. which isn't the case.
building off what calin said, the "programmable" polarizer (aka the liquid crystal matrix) is capable of rotating the polarization plane by 0-90 degrees. lcds are composed of 2 polarized panels (typically with polarization planes orthogonal to one another) with liquid crystals in between them. light enters through the first panel, gets polarized, passes through the liquid crystals (which are capable of rotating the plane of polarization), then passes through another panel and gets polarized once again. if the liquid crystals don't rotate the plane, no light is seen from that area (dead pixel).. if they do, then some light will get through (essentially the same thing that calin was saying.. except here the 2 panels aren't parallel. if they were, it'd give the same results).. the amount of light that gets through is dependant on the amount the l.c.'s rotate the plane, characteristized as brightness.
without the 2 polarized panels, the liquid crystals would be rotating unpolarized light by 0-90 degrees.. yielding unpolarized light rotated by 0-90 degrees. if 1 of the 2 panels are missing.. light would still pass through no matter what.. only difference would be that the light that comes out would be polarized, at a constant angle if its the back panel thats missing or at a variable angle if its the front panel.

as for backlight bleed through.. i have no idea.