As explained by Pan:

There are a few variations on the clothes pin game. They are all sophomoric and usually perpetrated by one or more film crew members trying to lighten the load (or boredom) of the twelve to sixteen hour day.

One variation is that just one poor schlemiel is targeted. The goal is to pin him as many times as possible without him noticing. As we saw happening to the first AD in FFofFII. (See? Kind of like Junior High School.) He probably noticed just about the time Peter slammed him into the wall or the side of the trailer. Ow.

I have a friend who has a variation on the single target game. The on-set electricians usually walk (or stand) around with large gloves hanging off their large tool belts. My friend thinks it's hilarious to try and fill up an electrician's glove with grapes or gummy bears or whatever is at hand. Don't ask me why.

Another popular variation is the lone pinner game. In this game one really really bored (and probably slacker) set worker has dozens of clothes pins he doesn't want and he just 'gets' everybody near him. Sometimes the pinner will write something witty on the pin first. The goal is to pin nearly everyone on set. Of course, after you've seen fifteen other people pinned, you'd be an idiot not to check your back, so this game doesn't usually last all day.

'Everybody near him', of course, doesn't include actors for the obvious reason that the pinner would be in deep doo-doo if the actor actually walked in front of the camera with a clothes pin stuck to him. Also, it usually doesn't apply to anybody else 'above the line'. A lot depends on how friendly the set is.