Focus is usually taken as having your full attention on a given task or goal. When people refer to someone being focused they feel that person is so concentrated on a specific task or goal they do not notice or get distracted by outside forces.
Is being so concentrated on a given task or goal a bad thing? It is when the outside forces are not even acknowledged. For an extreme example, take someone constructing a model airplane in their basement. A fire breaks out upstairs. The upstairs smoke alarm also begins going off and is very audible in the basement. The individual building the model is so focused on the task at hand, he has ceased sensing anything and everything that is not associated with his task at hand. Due to this focus the smoke alarm is not heard. Even the smoke that is beginning to billow from the top of the stairs is not seen until it reaches the work bench where it finally can be smelled. At this point it is to late to escape upstairs so hopefully there is a basement window that he can fit through to escape.
Now, if he wasn't so concentrated on the task at hand and was still conscious of his surroundings the smoke alarm would have been heard. The fire possibly could have been contained by him using a fire extinguisher. Now, due to that kind of focus there is most likely going to be severe damage and possibly the loss of his own life.
When you are involved in a disagreement your complete focus may lie in making your point. Due to that concentrated focus a small detail in their Body language or choice of words that could possible settle the dispute if noticed. Also, if you are involved in a physical confrontation it is easy to become focused on defending yourself against the attacker only. In that focus you may miss the extra help that shows up or you could miss other attackers joining in against you.
All of these examples showed situations where total focus interfered. It is great to be focused on goals and tasks but make sure it is not at the expense of your surroundings.