It has been a while since I put out a puzzle cache, and I figure it's about time. This was actually one of my first puzzle ideas. The acute among you will probably solve it quickly enough. But if you're having an off day, try looking at it from different angles. If the puzzle raises any flag with you, you're definitely on the wrong track.
For the FTF, there's a nano FTF geocoin taped to the bottom of the cache.
To save some time, check your solution with Certitude and get a location hint when you enter the correct coordinates..
Notes not related to the puzzle : Excuse my shoddy Photoshop hack job. Please replace cache in the correct spot to ensure proper adhesion. I searched long and hard for an in theme hide location, this is my 4th attempt. Take a look on the other side of the wall if you want to know why I picked this spot.