  We show that every groupoid C*-algebra is isomorphic to its opposite, and
deduce that there exist C*-algebras that are not stably isomorphic to groupoid
C*-algebras, though many of them are stably isomorphic to twisted groupoid
C*-algebras. We also prove that the opposite algebra of a section algebra of a
Fell-bundle over a groupoid is isomorphic to the section algebra of a natural
opposite bundle.
