Screenshot from the Zeus GUI element set from designshock.com
The single pixel stands out in the eyes on the upper icon and in the center of the q on the lower one; the groups of pixels working together in lines and color areas in all three; and, in the line that crosses the f you can see the misplaced pixels. Icons from Iconshock’s freebie sets (the errors in the last icon were purposefully drawn for this example).
The blue one shows the correct anti-aliasing, the green one shows a problem with the lines being blurry, the purple one the pencil-drawn jaggy corner, and the cyan one an overdone and sloppy anti-alias.
Edited screenshot to show the path to enter the window for the grid adjustments.
Here’s visible the grid with a point every pixel and every gridline spaced every 10px, the gradient layer with a vector mask adjusted to the pixels –as well as the path selected–, a raster mask to enhance the anti-alias and the pattern fill masked in.