Lucky Guess

 is a secret achievement/trophy in Deus Ex: Human Revolution. It is earned by entering the code to the bomb being used by Taggart during the side quest, "Smash the State", instead of hacking it.

Walkthrough
During the side-quest "Smash the State" (which you get after learning Sandoval's location either by convincing Taggart or by hacking his computer), you simply have to guess the code (which is 0000) to disarm the bomb instead of hacking it. You can also unlock this achievement by using an Automatic Unlocking Device to defuse the bomb.

If you have unlocked the Icarus Landing System, you can climb the ladder next to the bomb and perform a stun to disable it. Alternately, you can use a Gas Grenade to disarm the bomb.