I was just idly wondering about carbonation in cask beer. It's down to the person looking after the beer to ensure that enough CO2 is preserved in the beer to give it sufficient sparkle, and it doesn't always go right.
Sometimes beer is served much too flat. I don't mean not-through-a-sparkler flat, I mean absolutely flat. This happens more often when beer is served by gravity, but occasionally you get it on a beer engine too. You rarely get it too lively. Maybe this is because drinkers may complain if the beer is flat, but bar staff will definitely complain if it's too foamy?