Yes- you did something wrong. The gas and beer fittings are different, and you need to use the proper fitting with each post. You're going to have to try and muscle it, I don't know of any other real recommendation to get it off. I've heard of people breaking the QD's after this kind of stunt, so be sure to relieve the pressure when you are going to town. Best of luck, tho.
In my experience, when force-carb'-ing, there is really no need to push gas through the dip tube, and if you do so, be sure to use the BEER fitting.