Accidental and often grossly inappropriate mistyping of a left bracket instead of a right bracket when expressing a feeling of sadness and instead displaying joy.
The opposite can also apply, where instead of the intended joy, you express despair, sadness and woe!
Jane : I'm so upset, first my boyfriend left me, then my house got repossessed, and today my cat got run over by a lawnmower!
John : :)
Alvin : Hey Julie, I just got tickets for a weekend in Venice!
Julie : Really? :(