As from 2 days ago I'm not at home to give examples (on vacation.) It depends how you select the background & add or remove it. If you are using a mask to hide the background, then go back into the mask mode of that mask layer & recheck to be sure there is no imperfections in the color of the masked area. That's the easiest way I know of to find imperfections.

To do what you are doing, I'd initially duplicate the background layer. Then select the background area & add a mask to it so it vanishes. In same way I explained to you some time ago.

ie: select the layer the mask is on (the duplicate background), then click the Channels tab, select the mask layer so an eye is showing on it. This will then make the masked area appear a reddish color. Look for any imperfections where the red is not the same color (maybe a lighter or darker red.) You might need to expand the view to see the image in a larger view so individual pixels are easier to see.
Images under points #5 & 6 here are what I am referring to http://www.gnutellaforums.com/aid-di...oshop-cs5.html
Once you find pixels that are not 100% masked, you can paint over them with the pencil tool. Once you select the pencil tool when mask is active in view, the pencil colors will be either black or white. Black will add more masking & make those pixels vanish after you turn the mask layer off in Channels tab.
If unsure, simply paint over the entire masked area with the pencil so there are no semi-transparent pixels remaining.
I hope that is easy enough for you to understand.

On a side-topic, my CS5 is buggy & does not select or duplicate layers to 100% efficiency. I prefer using CS2 instead.