The plugs may have been swapped. They have the same thread into the keg, but the plugs are slightly different in size. This is the main negative on ball lock kegs. Most gas connectors have tiny notches in the base nut.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk