Claim: Friends gathered to view the video record of a newly-married couple's special day were instead treated to a literal episode of I Love Lassie.
Status: True.
Origins: I didn't quite believe this one the first time I heard it either. Yet here are a couple of 1994 news reports about it:
The 59-year-old lent his video recorder to a friend to film the wedding, but forgot to erase from the tape scenes of himself in sex acts with a neighbour's bull terrier named Ronnie. The man said the 10-minute film shown to the jury had been an attempt at trick photography and featured only simulated sex acts. He will be sentenced after psychiatric and other reports have been made available. Wedding guests were shocked when they sat down at a reception to watch film of the ceremony but instead saw sex scenes involving a naked man and a dog, a court heard today. Derek Jeffrey, of Saunder's Way, Camber, East Sussex, denied bestiality with a dog, but was found guilty today by a jury at Hove Crown Court which was shown a Jeffrey told the jury that he made the recording with the dog after he and a group of friends had watched a porn video involving sex between humans and animals. He claimed he was trying to demonstrate that such sex scenes could be filmed by trick photography and said his video showed only simulated acts. "When I sobered up I was disgusted and ashamed." Jeffrey said that when he lent his camcorder to a friend he thought the scenes with the dog had been erased.
A British man was found guilty on Tuesday of having sex with a dog after a video he made of the act was inadvertently shown to speechless wedding guests expecting to see a replay of a marriage ceremony.
Barbara "dog dazed afternoon" Mikkelson
Last updated: 26 February 2006
Sources:

Petras, Kathryn and Ross. Stupid Sex. New York: Doubleday, 1998. ISBN 0-385-48851-3 (pp. 24-25).