[liquidbrewing]

Did you force carb the beer at a higher psi?  The beer could be at 15. 

I'm betting that is it.

No, I didn't force carb at a higher psi.  Even if I did, this keg has been in the serving fridge for over three weeks now, it would have equalized. 

Is your CO2 tank getting close to being empty?

No my tank is not even close to empty, just got if filled a couple weeks ago.

Is your gauge in the fridge?

No the gauge is not in the fridge.

[Thirsty_Monk]

I wonder if you did not get backflow beer on your regulator.
You could take your regulator apart and give it a clean.

[liquidbrewing]

I don't see any residual beer in the gas line to the regulator though. 

Can a regulator just go bad?  Seems odd.

[punatic]

FWIW
When my gas bottle is near empty the regulator acts up.  Sometimes it over-pressurizes my kegs.
I avoid this by changing out the bottles before they go too low (I have a #5 and 2 - #20).