It is called fasciation, and can happen to stems, too.

It's more common then you think, but it doesn't seem that it is a stable genetic trait.

"Herbicides, insects and physical injury to the growing tip are reported to stimulate the occurrence of fasciations (Geneve, 1990). Fasciations also are reported to come about through spontaneous mutations (Gabillard and Pitrat). Conditions favoring rapid growth also favor the development of fasciations."

Here's one from someone else's camera...