If I were a scientist, articles like this one from Ed Yong would terrify me:
You’ve got a group of people with a mysterious disease, and you suspect that some microbe might be responsible. You collect blood and tissue samples, you extract the DNA from them using a commonly used kit of chemicals, and you sequence the lot. Eureka! You find that every patient has the same microbe—let’s say Bradyrhizobium, or Brady for short. Congratulations, you have discovered the cause of Disease X.
Don’t celebrate yet.
You run the exact same procedure on nothing more than a tube of sterile water and… you find Brady. The microbe wasn’t in your patients. It was in the chemical reagents you used in your experiments. It’s not the cause of Disease X; it’s a contaminant.
Apparently, this is turning out to be the case in a lot of experiments.