From across the Pond reports that a Barrister in England was removed from the jury because he understands how the system works. I'm not exactly sure why that would be a reason to remove him. Juries are supposed to have the collective knowledge and wisdom of the community in whatever permutation might specifically take form in a particular jury's members. On the other hand, the part about him knowing the prosecutor would be a valid reason to strike because of the possibility that an impression formed from that knowledge might change the lens through which the juror sees the evidence.
Mark from Sark linked to me a while back. Not sure if I noted that before.