The 9000 isn't immune to malfunctions, and has also got some quirks which may lead you to believe that it's broken. Some of the more common ones are listed here.
It is extremely common that the LCD display "delaminates", leading to blackened corners or edges. This can happen to both the LCD in the viewfinder and the data panel at the top. The LCD can be replaced, if you can dig up a pristine replacement; this can be very hard, however. Anyway, don't worry about further blackening, because the delamination stops at some point and won't spread further (spare due to drops and other violence). I have never heard of a panel going completely black.
Also known as "the camera is completely dead and won't do anything but blink the ISO number". This is a safety feature. When you remove the batteries, certain things will be "forgotten" by the camera, namely: exposure adjustment, film speed, and manual exposure settings. As a reminder that the film speed has been reset to the ISO indicated by the DX code (or ISO 100 if the DX code is lacking) and that you shouldn't forget to change the ISO, the camera will lock and blink (for example) "ISO 100" when new batteries are inserted. To fix this, just press the "ISO" button and enter the desired setting, or if the ISO number blinking is already correct, just push and release the ISO button. The camera now works normally.
The camera seems to work just partly, with the following symptoms true:
This probably just means that the aperture is stopped down with the Depth Of Field (DOF) preview lever. To reset the camera to normal operation, push the lever all the way to the bottom, or release the shutter to take a picture.
Provided that the picture isn't unsharp because of something common, such as too long shutter speeds leading to camera shake:
If you get a roll back which is almost completely dark, or extremely grainy if using print film, the camera's aperture control magnets may be broken. To check if this is the case:
If the aperture closes down to a small hole already at the fifth point, the camera has a problem and must be repaired. This is a more common fault in the 5000 and 7000 models, but may happen to the 9000 as well. The repair is unfortunately quite expensive; expect $100 or more.
I have seen this error being described as fatal on the net. I have never heard of it turning out to be a serious problem at all. Rather, the gears inside the film advance mechanism have jammed in some way. In most cases, what is needed to get a fully functioning camera is simply to (carefully!) operate the bottom film advance mechanism, namely the motor coupler. Use a flat screwdriver, and do not touch the center Phillips screw, it's the surrounding metal rings with "gaps" in it that should be rotated. Try both directions, apply semi-hard pressure until something happens.