Oil Slick

From the Super Mario Wiki

Mario near to an Oil Slick.

Oil Slicks are uncommon obstacles in the Mario Kart games. They primarily appear on standard road tracks, such as Mario Circuit, and cause the driver to spin out when he runs over them. As they are rather small and often placed at the edge of the road, avoiding Oil Slicks is not very difficult. Players can also jump (using a Feather) or hop over them.