Now this doesn't look like a very nice thing to do to someone. But I guess if s/he cheats, steals your wo/man, gets you fired, or breaks your favorite Star Wars collectible, one method of revenge is to Caltrops Tashibishi some S up. In this case, "S" would be tires. Cars, motorcycles, bikes, trikes, personal transporters--any of them are fair game for this 12-piece set of spikes that, no matter how you throw them, always land with a sharp edge pointing up.
Each Caltrops Tashibishi spike is finished in an inconspicuous matte black color and measures 2-1/2" overall. To be extra sure they go unnoticed after you ninja them into place. Just remember: karma's a bitch. Probably a bigger one than your ex.