Rovibrationally inelastic scattering of S-1 trans-glyoxal (CHOCHO, 12 modes) by H-2, D-2, and He is explored experimentally. Explicit attention is given to a state-to-state inelastic channel that is predicted to be forbidden on account of quantum symmetry restrictions in the symmetric top approximation for glyoxal (kappa = -0.99). This channel is 0(0), K' = 0 --> 7', K' = 0, where 0(0) is the S-1 zero-point level, 7(1) is the CHO-CHO torsional fundamental, and K' is the quantum number for angular momentum about the top axis. After S-1 <-- S-0 pumping of the initial state, inelastic scattering to the set of final states 7', K' = II that includes the K' = 0 channel is monitored by dispersed fluorescence. Relative cross sections for the rovibrational transitions are obtained for inelastic scattering by H-2 (E-c.m = 650 and 1170 cm(-1)), D-2 (750 cm(-1)), and He (750 and 1220 cm(-1)) to final states with K' ranging from 0 to 15. The predicted prohibition of the DeltaK' = 0 channel does not occur, but that channel is present with its scattering cross section reduced to only about half of the value expected if no symmetry effects were present. The partial relaxation of the quantum symmetry prohibition may plausibly be attributed to the small asymmetric top character of this near symmetric top molecule.