This is a quirk of Flash, which has caught me out a few times. If you're drawing a rounded rectangle with an internal fill colour and a stroke (outline) of 3 pixels, for example, then you get a rather jagged apperance between the fill colour and the stroke.
If you zoom into the shape, all looks fine and smooth, but zooming back to 100% the problem appears again.
To solve this, you must select the stroke (outline) of the shape - double click it to make sure all of it is selected. Then choose Modify > Shape > Convert Lines to Fills.
This solves the problem and your shape now looks smooth.