Whenever i get into a fire fight, and the left stick is pressed down (not the right stick) while firing. It does a melee attack and won't let me fire while ADS.
Just to clarify, i'm not pressing the right stick down.
I have it when I do a Stealth Kill out of Cloaking I can't hit anybody, shoot anybody or do anything "Aggressive." All I can do is run around with my powers like a sitting duck.