Camping In - S1-E4
Revealing mistake: Mrs. Slocombe enters her tent after telling Captain Peacock about the top window. Peacock then obviously uses that moment to stand on his tiptoes and peek in. You can see to the right of the tent the metal cart pushed by Larry Martyn discreetly peek out of the stage entrance and stop as he waits his cue to come on the set, to a rumbling noise. (00:13:35)
Camping In - S1-E4
Other mistake: The episode takes place on March 2nd, 1973 (date of the newspaper shown by Mr. Mash), but in Mr. Rumbold's office the sale chart for the month of March is already fully filled up.
Answer: That wasn't possible in 1972. Until recently squatting was a civil offence in the UK, not a criminal one, and removing squatters was a complex and time consuming business.