Seriously, test new skin care products somewhere other than your face

Yesterday  I decided to try a skin care sample (a moisturizer) on one side of my face, and use my regular one on the other side.  I thought it was a great way to see if one was better.  Well, it certainly was a great way to tell if there was a difference.  The difference is that apparently I'm allergic to the sample!  Yesterday I only noticed a patch of red skin on my cheek, but today I woke up with a BIG red patch that itches like crazy and has made my left eye swell.  Hydrocortisone is providing only slight relief.
Prior to this I've always tested any new product wherever I planned on using it.  I've had a few that caused burning or stinging, but never anything this bad.  So in the future I'll try new products somewhere less visible, and in very small amounts.  You might want to think about doing the same.