In one case a documentary serious about Scottish fishermen had to be subtitled because the accents/dialect were incomprehensible to general viewers in the UK:
What I find amusing is that as someone from the area in question it appears that most of the people being filmed are trying hard to talk "properly" - their real accents would be much thicker.
There's a similar show in the US about people in the deep south making moonshine, and I, a US born English speaker could not understand a word they were saying, even with the subtitles I had trouble understanding what word was what.
http://en.wikipedia.org/wiki/Trawlermen_%28TV_series%29
What I find amusing is that as someone from the area in question it appears that most of the people being filmed are trying hard to talk "properly" - their real accents would be much thicker.