  It is pointed out in a class of models with large extra dimensions that the
cross section of processes with virtual Kaluza-Klein graviton exchanges becomes
either much smaller or much larger by many orders of magnitude than what is
expected from that of the on-shell production of the Kaluza-Klein gravitons. We
demonstrate how the problem arises using a toy model. The cause of this new
problem lies in the fact that we do not have momentum conservation in the extra
dimensions. To search for the signal of the large extra dimensions with high
energy collider experiments, we need more care in interpreting the earlier
results on the cross sections of these processes.
