The big one
The number one mistake from my perspective, and the one that causes more resumes that I read to be tossed, is the good old conjunction. People have no idea how to use conjunctions. "I managed the set up of the new server system and am proud of the result." Wrong. Conjunctions conjoin two sentences that contain related thoughts. Each part of the conjoined sentence must stand on its own: "I managed the set up of the new server system" and "I am proud of the result."
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