I'm at my wits end trying to figure this out now, so it's time to consult the masses...
For the past 6 months or so, all of my homebrews, with the exception of one batch, have had an off flavor. It is a sort of citrusy acidic flavor. The beer tastes great when I siphon it into the keg; the flavors and styles have had a wide range from belgian strong ales, hefeweizens, to IPAs. Something is happening though when I keg the beer, refrigerate it, and force carbonate. I initially thought that I was over carbonating because I was among those that crank the PSI up to 30 then shake to carbonate it quickly. But then I more slowly carbonated a batch with 10PSI and got the same off flavor. The lone batch that didn't have this off taste was slowly carbonated and then sat refrigerated for almost a month.
One concern that I have is this: I don't have a liquid safety check valve that would prevent beer from backing up into my regulator and CO2 tank if pressures were out of balance. Is it possible that carbonic acid formation could give me these off flavors? However, I use standard Sankey kegs (not soda kegs) so I'm not sure that beer making its way back into the CO2 tank is a very plausible explanation anyway. Another thought is that thinking back, I believe the major change in flavors occurred when I last got the CO2 tank refilled. I had never gotten it refilled before around Charlottesville, so I went to the local paintball supply store. Could I have gotten a bad grade of CO2? Is there such a thing? I'm open to any suggestions or ideas. Thanks.