Megapubs- those places with 80+ beers on draft, can be wonderful places, full of interesting beers and general conviviality. But sometimes, as 90% of the patrons order the special or Budweiser, the more obscure beers sit on draft for weeks between pours.
Obviously, getting a glass of sour beer that's been sitting in the line is not going to leave you with an accurate impression of the brew. When ordering a craft beer, ask for a sample first. If it tastes funny, ask for a second one. You'll be more assured of a fresh pour after some liquid has flowed through the lines, and any bar that takes its beer seriously will understand this.