Now that you mention it, im not sure how it got co2 in the line either. It was leaking from the o-ring pretty badly, so I swapped the o-ring and lid with a new one. Still leaked, so I just set it aside. I will try the lube and see if it still leaks.
This one was deceptive, because it didn't leak at 20lb psi. When I would serve the keg, it would start leaking around 10 psi or less.