... There are two well known examples of gauge cancellations. ... The second example is the cancelation of double pole terms of t-channel photon propagator. ... For the above kinematics the photon appears very close to its mass shell ( ), hence this configuration gives a large contribution to the cross section. ... If diagrams of Fig. type contribute to your process, we strongly recommend to to set the 'Gauge invariance' switch ON (see Section ) to prevent the gauge symmetry breaking by width terms. ...