Probability & Statistics
It goes without saying that Probability and Statistics are important tools in science and engineering. However, before the early nineteenth century, probability and statistics were viewed as being distinctly different subjects. Landmark publications such as "The Law of Small Numbers" (L. von Bortkiewicz, 1898) and "Foundations of the Theory of Probability" (A. Kolmogorov, 1933) laid the foundation for widespread adoption of these tools in the scientific community.
Unfortunately, like many branches of mathematics, the powerful analytical tools of this field are shrouded in mystery by technical jargon and formulaic presentations. Indeed, there are an alarming number of instances where these methods have been applied incorrectly, sometimes due to an incomplete understanding of the underlying theory, and other times for less ethical reasons (e.g. see "How to Lie with Statistics", D. Huff, 1973; or more recently the "Science Isn't Broken" article (opens in a new tab) on "p-hacking").