There's a restaurant I've visited several times where the food is good, but the draught beer often tastes... off. It's hard to describe exactly how the taste is wrong, but the word "musty" comes to mind.
I know it's not the natural taste of the beer because the effect applies to beers I'm familiar with the taste of, such as Sam Adams Boston Lager and Yuengling Lager. It's can't just be a single bad keg, either, because it happened with different beers and on different days (weeks if not months apart from each other).
A friend suggested to me that the restaurant probably had dirty taps, but only based on my description of what happened (basically what I said above) and wasn't able to give me any more information than that. Is it possible to trace the cause of a bad-tasting beer to dirty taps just from drinking it, and if so, how?