The most amazing has to be Lee Clark, and anyone who KNOWS Clarky knows of his dislike to anything in red and white stripes. Even his regular jaunts down the Bigg Market with (then Mackem) Michael Bridges, when he would sing anti-Mackem songs.

And when he signed for Sunderland it took me months to get over it! Even as a Sunderland player, he gladly signed my Newcastle season ticket in a Durham bar when he was with the Sunderland team ... and rubbed it in Michael Gray's face!
Paul Bracewell and Stephen Caldwell; even back to the 1970s and Tommy Gibb and the legend - Bob Moncur. All have played for both teams.
And there looks to be a shock move from Sunderland in coming weeks who will make a bid for Damien Duff if they are promoted to the Premiership.