I'm on the Thieves' Guild quest where you have to find Mercer's Plans. I kept looking for it, but then I found out it was out of the map. Is there a way I can retrieve it? (I am on Xbox 360)
I've never gotten this to work, but it's supposed to be possible to walk through walls by holding a wooden plate, because of a bug in the game.
The Elder Scrolls Wiki has an article on doing this using the platter. I don't know if the wooden plate or platter works better, or if it makes any difference.
Once you walk through the wall towards the item you want, you can hopefully go pick it up. But if the item is out in empty space, you'll probably fall. Maybe you could just stick your head through the wall and then use telekinesis to grab the item. Or if it's close to the wall, maybe you could grab it as you start to fall. Once you've been falling for a few seconds, the game should reset your position to somewhere else in the house, rather than letting you plunge to your doom forever.
On PC, it's much easier to use
tcl in the console, like Jim Jones said.
Another thing you could try is to use a shout that causes items nearby to move, like Unrelenting Force. Maybe if you're standing in the right spot, you'll shout the plans into a location where you can go pick them up.
If you use the PC version, go ahead and press "~" to open console commands. Then type in below
togglecollision. This will allow you to fly through walls. Use to map to locate the plans outside the map.
You can also do a method of walking through walls using a plate. Wooden plates work the best. First hold a plate against a wall until it lines up. Then sprint through it. You might have to try a few times. See the other answer too.