Characterized by feeling that something looks fantastic on you which quickly dissolves the second you return home, relax for a bit and try on your new purchase.
When you put on whatever article of clothing you misguidedly bought and glance in the mirror, looking at an image of yourself wearing a horrified expression and something wildly unflattering...
That's when you know.
You've been a victim of Shoppers Fatigue.
Shoppers Fatigue is generally followed with Buyers Remorse. Can be remedied by returning whatever stupid garment you bought; may require a whole new shopping trip.
Look out for the signs and don't let it happen to you.