The Witcher 2: Assassins of Kings

This is a missable secret achievement available in Chapter 1. After you leave your ship you will arrive in a town called Flotsam. After you interrupt an event taking place in the town square, you'll meet the man in charge, Loredo. He'll invite you to his house at night.

When you go to Lordeo's house, Roche will join you, and the two of you will walk through Loredo's yard and up a staircase. A guard will turn you both away, and when you meet downstairs, you decide to sneak in. Right before you sneak, a man will approach you and say he overheard your plan. MAKE SURE YOU LISTEN TO HIM, as he then tells you to pick up a piece of the trap to capture karyan. The piece is called an iron frame and it will not update your quest log when you take it out of the chest (thanks WestsideSxE).

WITHOUT THIS TRAP, YOU WILL NOT BE ABLE TO GET THE ARTFUL DODGER ACHIEVEMENT.

After you agree to get the trap, Roche will distract a guard and you will be able to walk around the side of the building.

1. You will see a rock in front of you in the middle of the path. Go around the right side of it, and a guard will go around the left side. Wrap around, follow the guard, and take him out.

2. Continue behind the building. A guard is relieving himself. Take him out.

3. The door in front of you is locked. Turn right and you can climb up, then over the wall.

4. Follow the path stealthily, hugging the wall on your left, and the camera will pivot to show you an open window which you'll climb to in a minute. You will then see the next guard you need to take out.

5. Climb the ladder, hop the crates, hide next to the window and look in. After eavesdropping, climb back down to ground level, and watch out for the last guard on the left. Don't rush him, as he turns at the end of the path and will catch you. Let the guard walk to the dead end, pause, and walk back past you before stunning him.

6. You can open the door or go over the wall into the next area. Open the treasure chest and remove the trap component.

7. Climb over the wall and return to Roche. After you speak to him, the achievement will pop.

EDIT: Achievement Hunter put up a video. I'm not affiliated with them in any way, and they handle steps 1 and 5 differently than I did, but both solutions work. Their video is here:

This is relativity easy to do, but it's also easy to miss. Once you enter Flotsam you need to progress until Loredo wants to speak with you later at night. This will then lead on to you taking part in the "Indecent Proposal". Before you enter to meet with Loredo, speak with Roche below the stairs. A guy will approach and tell you some useful information, which then leads to Roche luring away one of the guards from the side of the house.

From here on you need to sneak around behind the house and collect the components of the Kayran trap that the strange spoke of. Make sure you also listen at the window to obtain some extra information.

The items you're looking for is located in a chest at the rear of the garden. You won't be alerted of the trap, just the wire frame needed for "Artful Dodger".

After clearing out the chests, head back to Roche and tell him of the information and items you gathered. This will then unlock after the conversation.