I had an interesting issue with my system in the last couple days. I kegged and carbonated a batch. I did so by shaking the heck out of the keg, with 30# CO2 on it. That method worked great for the previous batch. After a couple days of doing that, all I could get was a glass full of foam. So I warmed it, depressurized it, and chilled it again. Still, nearly all foam. When it settled, it tasted a bit flat. For a couple days, I messed with it, but it wasn't getting any better. I pulled the long dip tube out to make sure I put the o-ring on it. I did. But when I replaced everything, it poured nearly perfectly. All I can think is that the o-ring just wasn't seated properly, and it was allowing CO2 from the headspace into the pour, mixing beer and gas like a carburetor.