"Hey rube!" The call goes up along the midway. Perhaps a townie has gotten a little too fresh with one of the bally girls. Perhaps he's found one of the circus workers engaged in grift. Whatever the reason, the sound of the words "hey rube" meant there was going to be a fight and would bring in any circus hand within ear shot running.
Shows that had issues with towners or "gillies" had to be careful however, lest they "burn the territory," ruining both their own reputation and those of any circus that came to town in their wake and making it near impossible for anyone to book a show in the area. Often shows that allowed (or even advertised for) grifters or "fakirs" also had a "fixer," "patch," or "legal adjuster" whose job it was to smooth over any ill will by providing bribes to the local authorities in advance. Shows like those run by John "Pogey" O'Brien one of the most notorious grift show owners (and one time lessee of P.T. Barnum's circus) often sold the "X" or exclusive right to run a particular scam in exchange for a cut of the profits. Three card monte dealers, short change artists, pick pockets were all common types of grifters on the lot, and those were only the simple scams. Some shows even had grifters who engaged in complex long cons.
More legitimate shows, ones without authorized grift would also send a party like the "advance clown" ahead to make sure the local community had a good opinion of the circus by visiting hospitals, nursing homes and local businesses before the show's arrival. As small mud shows gave way to large rail circuses with regular yearly routes, it became increasingly important for a show to appear clean and above board for the sake of repeat business. To the "Sunday School Show" like Ringling Bros. and many other large circuses, grifting was a bane, one that their owners fought hard to prevent, even hiring local muscle to keep con artists off the lot.
In the end, be it at a circus with grift or an honest Sunday school show, woe to the townie who causes the words "hey rube" to be uttered. The words still hold the same power today on many circuses and carnival lots and have also been adopted into military parlance, used to signal a ship in distress and in need of air support starting in WWII. More recently, they've been adapted to social media by many circus workers, used as a call for help and support to other shows and circuses around the world and a way of letting other shows know about possible thieves and bad actors floating from show to show.