On my mouse, the rubber on the scroll wheel started to disentegrate. (No photos) but basically, touching the rubber would leave a black streak on your finger. The inside of the plastic housing was also black.

In addition to other debris, this caused the scroll wheel to stop working entirely, because the wheel registers scrolling by shining light through the clear housing through to the other side. You'll notice there are "fins" inside the wheel; the interruption of the light by these fins is how the mouse registers scrolling.

By placing a little outward tension on the center spoke, you can remove the wheel to clean the housing. Be careful not to lose the arm and the spring at the rear of this assembly.