A weird bug just recently. I fired a boomerang at a tank with only about 25 health left. The boomerang hit and killed the tank. On the same turn one of my teammates fired a digger at the tank (from some distance so it didn't hit until after the target had already died). The digger hit and I was credited with 88 damage from the digger as well. In the post-shot roundup I was credited with "88 damage with Boomerang", which is more than it's possible to do with the Boomerang. I gained 11Xp total from the shot. I don't know if the other guy also got credited with the Digger damage.
Well fwiw I think this must be lag related, since the servers are screwed right now. I just had the same thing basically happen in reverse - this time I was robbed of 6XP, so it all balances out in the end.