Symptoms
After a recent upgrade or skin change you find that customers that have previously visited your site are seeing a different skin than the one you want to use.
Cause
SkinIDs are cookied for 365 days, so any customer that has not cleared their cookies or forcefully been redirected to a different skin will get the SkinID from their browser cookie.
Solution
The best way to resolve this is to replace your old skin with the new one, rather than changing to a new skin ID.
1. Copy the skin_x folder that you are currently using, paste it back into the skins/ directory, and rename it skin_y (where y is the SkinID that you were using previous to the upgrade or skin change). You will now have 2 identical skins and customers with the old SkinID cookied will see the same skin as those users who have never visited the site before.