"r" opposites
Pairs of words that have a meaning which is "one-hundred-eighty degrees" different from each other when the letter "r" is added, such as "fiend" and "friend", "gem" and "germ", etc.
Perhaps the "r" opposites might have had a role to play in the infamous "cell phone static" call that got the totally-honorable-and-conscientious Enron officials in trouble --- you will notice that the moral-minded, "Ship the documents to the Feds" has no "r" letters in it, whereas the supposedly-uttered criminal statement, "Rip the documents to shreds" does indeed contain that letter a couple times, and has the exact opposite meaning.