Commercial beer is carbonated at the brewery, and is packaged fully carbed. The CO2 at the bar is used to push the beer through the beer lines to the tap. Sometimes if the bar has too high a co2 pressure it will (if given enough time) increase the carbonation level in the keg beyond what is proper for the style. For instance, If the bar setup is for highly carbonated American lagers, and you should tap a keg of English Mild, at first the Mild would be properly carbonated, and taste fine, but after a week or so would taste different. Sharper, more acidic perhaps, more bitter. Of course finding a proper English Mild at a BMC bar would be unusual.