Herbie rewrites floating point expressions to make them more accurate. Floating point arithmetic is inaccurate; even 0.1 + 0.2 ≠ 0.3 for a computer. Herbie helps find and fix these mysterious inaccuracies. site 
To demonstrate the workflow, We walk through bug #208 in math.js, a math library for JavaScript. The bug deals with inaccurate square roots for complex numbers. post
From the University of Washington's Programming Languages and Software Engineering group. site