I read about a LEO who made a traffic stop on two individuals. While talking to one suspect outside the car, the guy in the car shot at the LEO. Cop draws his weapon and returns fire. Doesn't even notice the guy next to him who draws another weapon and shoots him at point blank range.
Now that may be a poor example, because I don't know the exact angles involved and whether the LEO had tunnel vision specifically or if he just looked away at the wrong time. We can't be aware of everything all the time.
Then again, later on another law enforcement officer engages the same two suspects with a shotgun. He fires all his shells and does not score any significant hits (I think one glancing blow to the forehead). Then he empties his revolver and IIRC does not land any shots. He returns to his vehicle and begins to reload his revolver. They later found his spent brass in his pocket; muscle memory from his range shooting. He did not see one of the suspects sneak up to his vehicle and the criminal was able to shoot him at close range.
I am not an expert. I don't know how extensively these officers trained. But it's obvious they had enough training to make the pocketing of spent cases instinctual.
I am recalling this from memory so it may not be completely accurate. I do remember being appalled and saddened by the horrific circumstances that came to pass due to the officers not recognizing enough about their surroundings. Whether that is due to tunnel vision or bad luck I do not know. The whole story is on Wikipedia, and I'll look for it and post the link if i find it, in case I am remembering details incorrectly.
EDIT: I guess he did not pocket his spent brass; it was a rumor. http://en.m.wikipedia.org/wiki/Newhall_massacre