I think there is a bug with the adjustment brush....if I open an image with a sky, and use the adjustment brush, it causes bright areas in the sky to overexpose....but the problem is that I DIDN'T touch those areas....if you look at the two images, the bottom shows the much brighter highlights in the sky above her head, and you can see where I used the brush in the lower right side...so it shouldn't effect the area above her head....and the weird thing is, it does it even if I set the brush to lower the exposure.
I have tried resetting preferences as well as completely uninstalling Photoshop...and I even tried it on another computer on a fresh install and the same problem. It appears to be a bug.
I have also unchecked Show Mask and it still does it.
I noticed you are still using Camera Raw 5.4. You need the current version which is 5.6 from early Dec, 2009. I don't specifically know if what you are seeing is specific to 5.4, a bug or an issue on your system...but you really should update to 5.6 and check the current version. See the Camera Raw Product Page for update info.