Physicists create 'negative mass'
Physicists have created a fluid with negative mass, which is exactly what it sounds like. Push it, and unlike every physical object in the world we know, it doesn't accelerate in the direction it was pushed. It accelerates backwards. Ref. Source 3n.
The applications for this could be tremendous but I can see the military being the first customer. What are the practical applications of having a material do the opposite of what you expect besides offensive or defensive situations?