While doing the Quests Defeding the Strand (kill 10 scourge at the Argent Strand) and Parachutes for the Argent Crusade, I would attack scourge, an argent defender would assist, and then once the scourge died, the Argent Defender would turn on me and kill me.

http://img60.imageshack.us/img60/1815/wowscrnshot020909160339zu9.jpg

Screen shot of combat log

This is a known issue. Thanks.

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Maaven
Blizzard QA