People and vehicles at airshows and airports charged with the vitally important task of obstructing photographers trying to get pictures of the aircraft. They typically wear orange shirts and/or reflective vests to enhance their visibility. They are best known for making their appearance *just* when the most interesting aircraft of the entire event is about to pass by, remaining present until they have spoiled the photo opportunity, and then returning to their lair.
Sh*t! My photo of that F-100 was spoiled by the foreground obstruction crew!