Jeffry L. Rand
In 1936, Dale Carnegie wrote the world-renowned, aptly-titled, best-selling book on interpersonal relations, How to Win Friends and Influence People. In it, his first piece of advice was to never criticize any person, under any condition. In 2119, it became apparent that this held true for any alien as well, as Captain Jeffry L. Rand was to painfully discover.
Rand was the human captain of the starcruiser Far Voyager that made first contact with the VUX near Beta Mira in 2119 at the beginning of the war. Previously unknown to any species but the Yehat, Rand's objective was to make a favorable impression on the VUX in order to secure their allegiance into the Alliance of Free Stars.
Before even introducing himself on behalf of the human race, Rand made a fatal diplomatic blunder when he blurted an insulting remark to his lieutenant regarding the appearance of the VUX captain who had just appeared on the com-screen. Not knowing that the VUX had translated every word and that they were (and still are) an extremely vain race, all diplomatic relations were immediately severed before any ties were even sewn.
Needless to say, Rand's seemingly small affront cost the Alliance a valuable ally and gained them an adversary bitter to this very day. And there is no doubt Mr. Carnegie is still rolling in his grave.