I hope this is reproducable. This is what happened:
Changed team during a round (ex, You will be placed on the <something> team next round)
Killed the last surviving member of the other team (I believe there were 2 people on the opposing team and I was the only player on my team -- necessary detail?)
BEFORE I got the kill message, my team change took affect (I was now on his team)
Then I got the kill message, stating that I had killed my teammate (because I had changed teams). FFire was not enabled.
So yeah, not particularly important, but a bug nonetheless.