Collision-induced molecular energy transfer in the 12 700 cm(-1) '4 nu(CH)' rovibrational manifold of acetylene is studied by infrared-ultraviolet double-resonance spectroscopy. As in previous work, there is evidence of (formally forbidden) odd-numbered changes in rotational quantum number J. Such 'symmetry-breaking' processes are invariant to choice of fluorescence-monitored vibronic band. The prominent (10300)(0) J = 12 rovibrational state appears to be a likely gateway and is rationalised in terms of a combination of quasi-resonant Coriolis and Stark mixing. An unusual collision-induced quasi-continuous background, underlying the discrete rovibrational states of the 4 nu(CH) manifold, offers a mechanistic key to the apparent symmetry breaking.