I received a HP DV6500 laptop from a client with the complaint that the screen was corrupted. The display looked offset and flickered and jumped, sometimes showing white and black blocks and lines.
This is often a symptom of a faulty NVidia chip, for which the easiest cure is to replace the motherboard. (Some solutions involve heating the chip to melt the solder and re-establish its connection to the motherboard.) Before taking such expensive and troublesome steps, it made sense to do more troubleshooting.
I connected an external monitor and the display looked fine on that, so I figured that the problem in this case lay with the LCD or its cabling and not with the chip.
I opened up the laptop (you can download a service manual from HP's website) and checked the LCD data/power cable at several points. Nothing changed when I reconnected the cable at its attachment point to the motherboard or when I wiggled it, so I went inside the laptop lid. At the point where the cable joins to the LCD itself, it was loose! I reconnected it and taped it more tightly. Problem solved!