"First do no harm" is as true of our fitness practice as it a principle of the medical community; we should always evaluate new diets and exercise fads with this in mind. Example: An article says cinnamon may help control blood sugar and weight management, so you are thinking about adding it to your diet. There's no harm to it, so why not? Go ahead and add a teaspoon daily. But what if another article says to cut out a particular "evil" traditional food group like potatoes, eggs, whole grains, and certain forms of protein? Then you may want to carefully consider what harm could come from avoiding whole categories of certain foods - especially ones that provide essential nutrients without consulting your doctor and/or professional nutritionist.
For exercise, this concept holds even more truth as we evaluate new fads or consider approaches to get into or stay in shape. What good does it do you to over-exercise if you could potentially injure yourself? An injury may mean you can't exercise at all for months or need surgery with its associated recovery period! The immobility required for one part of your body to recover may mean the rest of your body is doing without the vital mobility it needs. The result could be even more body breakdown - and no one needs that! Sometimes, the negative impacts of doing somethng we know could potentially harm us won't show up immediately, but will in later years as wear-and-tear and pain issues.
In the yoga community, this concept is called an "ahimsa," or non-harming. We all have inner wisdom and "body feedback" when we tune into it, but sometimes we ignore it for the advice of an "expert," or to try to break through a fitness plateau or to explore a shiny new fad. Add more joy and less pain to your fitness by using the "first do no harm" filter before proceeding.