[LHM]

My textbook says, "There are a few exceptions to the rule about double bonds: in a few instances both bonds of a double bond are pi bonds." Just wondering, which exceptions are these, and why are they exceptions?

[Schrödinger]

The only 'existing' exception that I have come across so far is C2. But it is quite easy to find some more, if you try to use Molecular orbital theory

http://en.wikipedia.org/wiki/Diatomic_carbon