“Hey, what the hell are you doing?” screamed “Mad Men” star JON HAMM – who’d spied a young female extra snapping pants-off pics of him! 

Said my on-set source: “Jon had to redo a quick scene and needed to get back into the pants he’d worn before. Rather than send him all the way back to his trailer, the wardrobe folks had him change in a makeshift dressing room just off the show’s main set.

"Then this sneaky girl extra ran up some stairs to an area that overlooks everything and – as Jon dropped trou – she started snapping away. But he heard a noise, looked up and – caught with his pants down – flat-out exploded, yelling, ‘You come down here RIGHT NOW!’”

The 20-ish lass tried to duck and run, but was quickly caught by security guards. Zipping up his pants, the star made a beeline for the girl, then stood over her as she erased every R-rated pic of Hamm’s hams.

Little Miss Horndog was then fired on the spot and escorted off the set by security.