The 52-year-old antiques expert was taking part in the third day of a Scottish road trip with Catherine Southon when he made the damaging blunder.
Starting in Dundee before travelling through Forfar and Inverurie on their way to Aberdeen, Mark tried to source an item “in need of minor restoration”, but left it needing further restoration after dropping it on the floor from standing height.
Voicing the show, expert Tim Wonnacott said: “Inside Clepington Antiques, owner Derrick is standing by to help. With Catherine snapping at his heels after the last auction, Mark’s decided to get all tactical on us.”
Speaking to the camera, Mark pulled the clock out of a sealed cabinet and explained: “What I’m trying to do is… we’re going to a salesroom, so I’m trying to find things like this which might need a bit of restoration.
Antiques Road Trip havoc as Mark Stacey BREAKS rare clock after floor drop: 'No!'
That’s not recommended. You shouldn’t actually do that
“But it looks as if it has just come from a house clearance, but there’s no price on that.”
Turning the item upside down, the clock face fell out and rolled across the floor, leaving Tim to joke: “Oh, that’s a good tactic Mark.”
Red-faced, he scrambled to find the clock parts on the floor to piece the item together again before the shop owner returned.
Fumbling with the rare item as the store owner was nowhere to be seen, Mark said: “Can I just point out? Can I just say something to you at home? That’s not recommended. You shouldn’t actually do that.”
Antiques Road Trip havoc as Mark Stacey BREAKS rare clock after floor drop: 'No!' Antiques Roadshow's most valuable finds Sat, April 23, 2016
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Snapping at Mark’s handling of the antique, Tim added: “No you should not! Unless you want to pay the full ticket price for it. Thankfully though, nothing’s broken, but best find something else to look at, eh?”
It came after Mark continued his animosity towards Catherine after storming off earlier this week.
At the start of today’s episode, he snarled: “I’m going to be very, very tactical. Oh yes. The gloves are off. The gloves are off Catherine,” promoting his rival to gasp: “Are you? That worries me a bit.”