skimmer overflowing

That's actually pretty common. A great deal of the time it happens because of oil. If you noticed that the bubbles were larger than normal and seemed to just boil over rapidly, that's what it is. A lot of times suppliers will use a very thin coating on the salt containers to make them air tight to keep the salt mix fresh. Unfortunately, the coating sometimes comes off onto the salt and when you mix up, there you go. It bubbles up like mad.

It doesn't do your tank any harm at all, but it does react VIOLENTLY with skimmers. The cure: Simply turn your skimmer off for a day and a half. The compound evaporates and everything goes back to normal.