Astronomers have discovered "a strange spiral galaxy show it has a pair of arms winding backward compared to the typical direction for most galaxies":

Most spiral arms observed so far tend to trail in the wake of their galaxy's spin, meaning they wind in the direction opposite the rotation. The strange galaxy, known as NGC4622, lies 200 million light years away and has a large outer arm pair that winds clockwise.

Byrd and his colleagues analyzed a 2001 Hubble Space Telescope image of the galaxy and found a previously hidden inner pair of arms winding counter-clockwise. Whichever way the galaxy happens to rotate, one pair of arms ends up turning in the unusual direction.

"Contrary to conventional wisdom, with both an inner counter-clockwise pair and an outer clockwise pair of spiral arms, NGC4622 must have a pair of leading arms," Byrd said. "With two pairs of arms winding in opposite directions, one pair must lead and one pair must trail."

One possible explanation is that this galaxy have have "devoured another smaller galaxy". Here's the image.