I was doing some color correction recently and noticed there is a bug in the Windows version of Photoshop CS4 when using the color sampler tool shortcut that places a point on the Curves Adjustment layer.
Using the colour sampler tool in Photoshop CS4 you can select a point on an image, which places a marker and allows you to read the RGB relevant numbers on the info palette. If the curves tool is open and you CTRL -SHIFT select the marker again, the point is placed on the individual Red, Green, and Blue curves that relate to the colors of the marker.
This should also work when you are using the Curves Adjustment layer, and I have watched people use this shortcut on Mac machines without problem.
However it does not appear to work for the Windows version of Photoshop CS4. Nothing seems to happen when you use the CTRL-SHIFT select function when the Curves Adjustment layer is selected.