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NASTY NEIGHBORS ON "20/20"

Airing Friday, September 20, on ABC

Neighbors terrorized by a bully in a bulldozer; a tiff over trees that became a deadly turf war; a high-tech neighborhood watch system... "20/20" pulls the blinds back on the nastiest neighbors around, FRIDAY, SEPTEMBER 20 (10:01-11:00 p.m., ET) on the ABC Television Network. Anchored by Elizabeth Vargas and David Muir, reports include:

· Bulldozer: What would drive a Port Angeles, Washington man to destroy his neighbor's property with a bulldozer? Jay Schadler reports on the ultimate neighbor nightmare, including exclusive video of the unbelievable act in progress.

· Neighbor From Hell: It seems everyone has a nasty neighbor story, but Kim and Greg Hoffman say theirs takes the cake as their neighbor Lori Christensen, a single mother and former Girl Scouts leader, made it her mission to ruin the Hoffman's lives. Her brand of terror was wide ranging - from verbal harassment, berating Kim in front of her children for being a recovering alcoholic, to incessant videotaping as the family comes and goes; from unwarranted calls to police and housing inspection authorities, to covering her garage door with bizarre and degrading signs - one that read: "I Saw Mommy Kissing a Breathalyzer."

After months of legal negotiations, Christensen had agreed to plead guilty to violating a harassment restraining order and receive 10 years probation and a 10-year "stay away" order that would have kept her out of her house for a decade. But just last week she was back in the headlines again when she submitted a motion at her sentencing hearing to withdraw her guilty plea, suggesting it had been coerced. Amy Robach sits down with Christensen to discuss the latest legal twist in the ongoing saga.

· Nextdoor.com: It's billed as a customized social network for individual neighborhoods - a way to connect, to problem-solve, even to fight crime. Nextdoor.com is a high-tech neighborhood watch, where neighbors can post personal observations and security cam images. But is it also just another way to gossip or to spy? Nick Watt reports.

· A Lethal Line in the Sand: Matt Gutman reports the tale of two neighbors...one a well-known do-gooder who devoted his life to helping children and the elderly, and the other, an intellectual, an inventor, and immigrant carving out his American dream. Neither of them are magnets for trouble. So how does one of them end up in jail and the other in the morgue over bushes that separated their homes?

David Sloan is senior executive producer of "20/20."