How to Prank a Rubiks Mastermind
Take Advantage of Their Pride
Things I've observed over the years with the cube...
If you peel off the stickers, they can easily be mistaken for the color white. Easy stuff.
With my sets of moves, you can always reduce the unsolved cubes down to zero or two or four... never just one. So simply take apart a 3X3X3 cube and rotate a single edge piece and put it back together. They will spend a while trying to solve a single goddamn edge piece wondering why the hell their favorite moves aren't fixing the problem. You can tell how good they are at solving the cube by how fast they figure out something is wrong with it.
Now the above prank won't work if you're spinning an edge on the 4X4X4 cube. The mechanics of the object as well as the dynamics of the cube make it impractical to rotate edge pieces. However, the same dynamics apply to the corner pieces of the 4X4X4 cube as the 3X3X3 cube. So simply rotate one of the corners. No move exists for any cube that rotates just one corner. So if they solve the one, they'll unsolve another.
Don't put stickers back on if you peel them off. First, everyone that can solve the cube hates it when you peel off the stickers. It's like defacing something holy to them. If you do decide to peel off a sticker, putting it back on is the biggest give-away that something unfortunate happened to their sacred cube.