It has happened to me a few times, but it is a rather rare phenomenon.
And, I've also been granted kills for which i have done very little (once I was landing when a 51 bounced me. The ack killed him after he pinged me some), so I feel all in all the balance is quite good.
It's a minor irritant to me, and one I can live with since it is rare.
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StSanta
II/JG2