Patch Test. Always.
Unfortunately, many people learn the hard way that there are two things you should always patch test without fail. The first is hair dye. I don't care how many times you've dyed your hair, an allergy can suddenly develop at any time, so you must patch test every single time you colour at home (even when you're using the same brand and colour) and every time you visit a new salon (this goes for lash and brow tints too). We've all read the horror stories, so why take the risk?
It's also a good idea to patch test anything you use on your face, particularly exfoliators and anything with active or particularly strong ingredients. It may seem like a waste of time, but it's not just those with sensitive skin who suffer. I have very tolerant skin, but I've had three bad reactions to three very different products in the past. And beyond testing for an allergic reaction, a patch test also lets you see how well a product works with your skin type before you use it all over your face. You may not get a rash, but what if your skin goes flaky, dull, or breaks out? Catching that on a small area on your jawline or cheeks rather than your whole face saves a lot of heartache!