This usually happens in search and destroy. I seem to constantly encounter some person that will make some excuse every single time they die.
-Pretty much any weapon/perk/equipment in the game is over powered.
-The person is "camping" when they are just playing the objective by protecting the bomb site.
- "hard scoping" which translates to using a sniper rifle for its intended purpose.
Countless others, but those are the most common I have seen. Their logic is that the person who kills them is the one who sucks. An example being one of these miscreants was complaining about a guy going 11 and 1 while he was around like 2 and 6.
If they get overly annoying they can always be muted, but sometimes I find them rather amusing at what lengths they will go to claim their superiority.