I bought the laptop second hand and it worked perfectly fine for two months--then one day when I was taking it out of the closet I keep it in at night, I accidentally bumped the corner of it on the door frame. The monitor had never acted up before then so I'm thinking that may have done something to it (and yes, I know correlation != causation, I'm just saying that might be the reason it's acting up.)
Anyway. Every once in a while, but with increasing frequency, the LCD monitor of my laptop (Compaq Armada M700) will slowly turn this weird purplish-grayish-white color, and if I don't turn it off right away, there'll be a large black bar going across the top maybe 1/3 of the screen. The computer will freeze and I won't be able to do anything, so I have to do an emergency shut-off, wait a minute or two, and then turn it back on, and it will usually work. It usually only does this when I've been on the laptop for a few hours, and it has never happened when I'm on Firefox (although that's not to say there won't be a first time), only when I'm on the desktop. It seems to like to happen when I disconnect the router cable from my laptop (I don't have wi-fi). Sometimes it'll just do it out of the blue though. And once or twice it's done it when starting the computer up, but only after it had been on for a while and I'd turned it off.
It's not doing it right now, obviously, as I'm using that laptop. It's driving me crazy though.