I'm guilty of this just like everyone else.
Not using your radar, comming round a corner, them emptying a full clip into your teammate cause they scared you.
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I done that too... bad guy came around the corner I shot him up he died... I was in another corner to reload. I hear someone reload around the corner I thought i was slick and chucked a nade against the wall to get him... it was a team mate and he ended up killing me on accident, because we got into a gun fight after the nade.
lol Newb quotes:
"i didn't know you can defused with out a kit!"
"Lemme cover your back by just standing behind you!"
*chucks a grenade by your feet* "Take cov~ oh my bad."