Women, when in a relationship take fights very seriously. They argue with all emotions and posses fragile sensitivity level. What you consider to be a joke may lead women to blast. 
When they are angry or depressed, being a man you need to be cautious while talking. There are several things a man should never tell a woman during the fight.
Here is the guide for every man.

Never ever say to relax
In between a fight or in the hot argument aking her to relax can make her angrier. It literally conveys that you are indirectly indicating that she is out of the mood and out of control. Never try to prove that you are ok and the lady is abnormal.

No expressions of love or sex
Expressing your feelings during a fight is a big NO. You will get an immediate denial and it will only help to lengthen the fight. The direct mentioning of sex during a fight may worsen the condition than ever. Sex is not a source of calming down for women.

Avoid words hint her past life
Mentioning her ex-boyfriends or ex-husband while you fight may make her angrier. It just irritates the woman and she may go to sudden extents of anger and hatred.

Do not go on apologizing
Apologizing with a fight women may bring back the negative results. Silence is a better option to calm her down. Try to protect your arguments in the easiest possible way.

Do not pretend as if you did not pay attention
Trying to divert her attention can make her more furious. Just pretend that you are listening with more concentration. 
With your body language and gestures, just convey that you are grasping what she says.