Way I always understood it it was like the Japanese l/r thing. The wrong guys just couldn't pronounce the sh.

There was a Frisian/Hollander(?) thing. The Frisians would make the suspect say "brod, buter ond grene chiese, ond wat dat nat seyse ken is kin uprechte Friese." Or something like that. There were sounds in there that a Hollander (or whoever it was) just couldn't wrap a tongue around. I got the line from The Story of English by Mario Pei. He was demonstrating the closeness of English and Frisian.

I met a Frisian on the train from Flagstaff to Chicago once and spouted the phrase at him. After he recovered from the surprise of finding someone in America who knew any Frisian at all and corrected my pronunciation he gave me the full story.