Image remnant "ghost" on layer mask

I'm having a problem with the layer mask in photoshop CS (Win)

I'm using a layer mask as usual to create some rounded corners on a photo. Trouble is, when I try to paint the selection, it leaves behind a faint "ghost" of the image, and nothing I do seems to get rid of it.

I have all my opacities set to 100%

I just can't figure out why this is doing this. Is it a bug?

Is your black at 100% (R=0, G=0, B=0) or is it a shade i.e., almost black?

Well since it's a mask, it defaults to black/white. I haven't changed that (to my knowledge) so as far as I know it's defaulting to black.

One thing I just noticed that seemed to fix the problem...

If I alt+click on the layer mask itself to expose the "mask channel", and then do another fill of the selection, it seems to get rid of the ghost.

Why wouldn't it work in non-mask channel mode? Is this a "feature" or a bug that's new with CS?

I second Ho's question, as it hasn't been answered.

Dennis, I don't think the color is the problem. If the default black was set to something less than "real" black, successive applications wouldn't change the opacity. Now I'm leaning toward something being awry in the brush dynamics (wet edges?).