My pet peeve usage: "allegedly", for convicted criminals. After the trial there is no more allegation; they've been convicted. Their participation in the crime is legally a fact.
But if its your pet peeve, you should read the New York Post, where they drop the "allegedly" and go right to calling them "scum" "perps," "villains," and "thugs." :)
I think the saddest case of the euphemism 180 is the word "lady". In the word's true sense, it's a huge compliment for a woman to be called a "lady", but it has been cheapened by the (originally ironic or euphemistic) diminutive and insulting uses ("bag lady", "dragon lady", "cleaning lady").