One of my housemates is from Zimbabwe, and for the first few weeks it was really hard to understand what she was saying, I had to ask her to repeat everything two or three times 'till I got the hang of it.
Then there was the one time my family went to the Lake District in Cumbria and asked someone for directions. None of us understood a single word he said, we just sort of smiled and nodded confusedly.
Oh, and closer to home there's the Black Country dialect, also known as Yam Yam. Just...for the love of God, do not go to Wolverhampton. The accent's pretty bad as well.