In Photoshop CS4, Adobe introduced some new features to the Clone Stamp Tool, and in the process have managed to confuse more than a few people. The Clone Stamp Tool paints over the image using pixels that it draws from a “source,” and starting with CS4, this source is displayed live under your brush (this is a feature that migrated over from the Vanishing Point filter). The result is a colored blob that seems to undulate and flow as you move the Clone Stamp Tool brush over your image, and this colored blob is the source of confusion for many people. See the image below as an example:

The colored blob is actually a preview of what you will paint, if you click the Clone Stamp Tool in that spot. It is useful for lining up the clone source with the target, but if you don’t like it, you can easily return to the old behavior. Start by opening the Clone Source Panel (Window > Clone Source).

See the check box for “Show Overlay?” Just un-check that, and your preview disappears – your Clone Stamp Tool is now reset to behave exactly like it did in Photoshop CS3 and earlier.