  Although it has long been known that the proper quantum field theory
description of critical percolation involves a logarithmic conformal field
theory (LCFT), no direct consequence of this has been observed so far.
Representing critical bond percolation as the Q = 1 limit of the Q-state Potts
model, and analyzing the underlying S_Q symmetry of the Potts spins, we
identify a class of simple observables whose two-point functions scale
logarithmically for Q = 1. The logarithm originates from the mixing of the
energy operator with a logarithmic partner that we identify as the field that
creates two propagating clusters. In d=2 dimensions this agrees with general
LCFT results, and in particular the universal prefactor of the logarithm can be
computed exactly. We confirm its numerical value by extensive Monte-Carlo
simulations.
