Just wanted to double check a couple of things for my own sanity! I am looking at scattering amplitudes for 2 partons going to two partons with the emission of a Z boson (eventually decaying to e+ e-). I have used madgraph to plot the possible tree level Feynman diagrams for a couple of processes and I'm a bit surprised by the output...See link below for the output diagrams.
Initially I thought that diagram 2 cause of the virtual anti-down quark (displayed as d~). It looks like a d quark has radiated a Z boson which changed the d quark to an anti-down quark which in turn radiated a gluon which changed it back to a down quark. I am 99% sure this is not allowed as the Z can't change q's to qbar's.
An alternative way of thinking about it is that the u quark radiates a gluon which then pair produces a d-dbar pair, one of which is final state particle but the other one goes and annihilates the incoming d quark to give a Z...But I am not certain that this is allowed either!