Proverbs 26:2

"As the bird by wandering, as the swallow by flying, so the curse causeless shall not come." ~Proverbs 26:2 (KJV)

Two Commentaries comment:
"He that is cursed without cause-the curse will do as little harm as birds flying overhead."-from the commentary of Matthew Henry

"The curse that is unjustified never does anyone any harm. It is a refutation of the superstition that the righteous could be hurt by the malicious curse."-from the commentary called "The New Bible Commentary: Revised" put out by Eerdmans

From me:
Since the rest of Proverbs 26 gives lessons about the fool, I'm guessing verse two is also referring to a fool-it is implied. Perhaps a fool tries to put a curse on someone (not sure if this means cursing at them, or a spell), but the fool's curse is ineffective at hurting the person who is undeserving of the curse. This fits because in verse four it tells us what to do if a fool does try to unjustly curse us. It says, "Answer not a fool according to his folly, lest thou also be like unto him."

To me this means, when others curse us, do not return in kind. That sounds similar to Matthew 5:44, which says, "But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you."