I have noticed this too,

Another thing ive seen is a team mate and I could be fighting one person the enemy will die and give my team mate and I a kill on that one person. Giving the enemy two deaths when it should have been one. Doesnt always happen but have seen this a few times.

Also ive seen team mate kill teammate. Im not sure what is causing this, a bugged skill inflicting damage on both teammate and enemy? I have kill my teammates many times.