I would check the coupler seal and the seal on the keg itself. There should be a rubber gasket on the coupler and another one above and around the ball on the keg. Check those seating surfaces, that is the only was I can think of to get beer in the CO2 line, but it is definitely a unique problem. I would check em out and give em a good look over, when you get it figured out, let us know what it is, I'm pretty curious.
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