During a blood transfusion if you get a fever and no other problem is this a dangerous transfusion reaction?

Usually no: If the only adverse reaction is a fever then the reaction is unlikely to be dangerous/significant - patients recieving transfusions are sometimes pre-treated with tylenol (acetaminophen) and Benadryl to prevent these minor reactions from being too symptomatic.

Maybe: If there are no other symptoms it may be a reaction to something else. However, your physicians can easily investigate the presence/absence of a reaction, and the transfusion should be stopped at the first sign of one.