I have just started on my first day of Photoshop's 30 day trial. So far it has been fantastic, but I ran into a show-stopping selection problem. Photoshop is treating pixels as if they are selected when they clearly aren't. I have zoomed in 800%, and sure enough, the pixels are not selected. However, when I copy the selection, those pixels come with the copy (I have one layer). The stray pixels are scattered around the real selection, but, again, are not selected. To prove that they PS thinks they are selected, I used the 'subtract selection' option in the general area of some of the pixels (where nothing is selected). When I copy, the pixels in that area now not copied. Does you know why? If no one has encountered this problem, I can upload screenshots of what I mean to prove that I'm not crazy, but that will have to be tommorow because I'm on 56k right now.

The marquee only shows you selection up to 50%, IIRC.
If you want to clearly see which pixels will be selected and which will not, you can switch to the Quick Mask mode, where a layer of red will show you your selection much better.

What you are describing could happen very easily if you use Feathering on your selection, or as the previous poster mentioned, if you have AntiAliasing checked.

It's not really a problem, it's just a concept you have to get used to, i.e. a selection is more like a mask than anything.
Try to see how it works by switching to Quick Mask mode, then start using your normal painting tools to "paint" the selection you want. Use shades of gray, BTW.
once you have messed with the Quick Mask, simply switch back to Normal selection, and you will see a resulting selected area.
Notice how it doesn't necessarily show ALL the pixels that you painted as selected.

I hope this is clear enough, but if you don't understand, ask again, I'll be happy to go into more details (I can't really show you right now, coz I dont have Photoshop on this machine).

Good luck !

Thanks a lot, that was the problem!