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.