Where is your barrel pointed?
The first thing that should be going through your head at all times is tactics. If your tactics are poor you can get yourself, your partner, or other responding units hurt.
The topic of clearing a house for a perp is very important. The other day we were called to a possible man with a gun inside of an apartment. When the responding officers knocked on the door, the owner of the apartment claimed there was nothing going on inside and offered to let them come in to check. There were about 5 cops who entered, all with their guns drawn and all pointing their weapons at each others backs while clearing the apartment.
As far as I am concerned, the only person who should have his gun pointed forward is the first person in line, unless someone else breaks off to clear another room. When guns are involved, everyone should be asking themselves, "Where is my gun pointing?"
Another tactic I have seen took place during felony car stops, where everyone had their guns drawn but no one was thinking about tactics and the position of their barrels. In a situation like that, you should always be thinking about crossfire.