Stitching up a random person (they can be on the bus, walking down the street, whatever) whilst recording the act with a video camera phone or taking pictures using a camera phone. It's a very unfortunate Cambridge phenomenon. Stitch Ups are usually made by yoots (ghetto versions of chavs) thinking they are rudeboys or people after attention. A happy stitch is made by the unfortunate person's bag or property being completely trashed. Victims are usually boys around the same age as the attackers, but it's not unknown for victims to be older, or even women. Sometimes the tables are turned, as there are pictures and videos going around where the attacker is happy stitched or just plain stitched up by the victim. More power to the victims, I say. The act is totally random, so be on the lookout for a group of pussified yoots or attention seekers pointing their camera phone (that their mum bought) at you or your property.
Boy 1: "Dude, let's happy stitch that innocent boy at the front!"
Boy 2: "Put anything you can find in his bag, and take a picture"
Later that day:
Stitched up boy: "WTF is this shit in my bag man!"