Beware of Greeks bearing gifts
Don't trust your enemies.
An allusion to the story of the wooden horse of Troy, used by the Greeks to trick their way into the city. It is recorded in Virgil's Aeneid, Book 2, 19 BC:
"Do not trust the horse, Trojans. Whatever it is, I fear the Greeks even when they bring gifts."
Of course that English version is a translation. Another translation, by John Dryden, has "Trust not their presents, nor admit the horse."
The same thought was also recorded by Sophocles (496 - 406 BC), in Ajax:
Nought from the Greeks towards me hath sped well.
So now I find that ancient proverb true,
Foes' gifts are no gifts: profit bring they none.
The Classics are no longer widely taught or read, so this phrase is now little used, although it was resurrected in a sideways reference during a 1990s copyright dispute. There was considerable discussion then, in Internet chat rooms etc., regarding the company Compuserve, which owned the copyright to the GIF image format, and their possible intentions to restrict its use. Some people feared that they might be taken to law by Compuserve if they received and viewed GIF images without permission. The phrase "beware of geeks bearing gifs" was coined to sum that up.
See also: the List of Proverbs.