Your diction is simply your choice of words. There is no single, correct diction in the English language; instead, you choose different words or phrases for different contexts:
To a friend
To a child
To the police
To an employer
All of these expressions mean the same thing -- that is, they have the same denotation -- but you would not likely switch one for the other in any of these three situations: a police officer or employer would take "screw-up" as an insult, while your friends at the bar after a hockey game would take "oversight" as an affectation.