When it comes to NIMBY opposition that derails plans for new housing, it’s not just the neighbors raising a stink.

Sometimes it’s rival developers worried about the competition that are to blame for purposely gumming up the works.

That’s apparently the case with Arthur Leon and JACE Boston, a major South End property owner who now finds himself staring at a $5.8 million court judgment.

Last week, a Suffolk County Superior Court jury found that Leon and JACE had abused Boston’s permitting approval process, and in one case the criminal justice system, in a bid to obstruct plans for a $20 million high-end condo building next door to one of their South End properties.

The attempt failed, but not before delaying plans for what are now the Jordan Lofts at 477 Harrison Avenue, at one point driving the developer, John Holland, to the brink of bankruptcy.

The verdict capped more than a decade of dueling lawsuits over Holland’s ultimately successful plans to convert an older commercial building into loft-style condo units, which opened in 2016.

Still, complicating matters from the start, one of the walls of Holland’s Harrison Avenue building was connected by a seam with that of Leon’s old Washington Street industrial building. When Holland first informed Leon of plans to redevelop his Harrison Avenue building, originally built in 1913 as stable for the old Jordan Marsh & Company, the fellow developer and property owner asked him to wait a year.

Leon claimed he had his own plans for a big mixed-use project, Holland testified.

In fact, Leon was particularly concerned about Holland’s plans to build pricey penthouses, arguing it would attract rich people who would then object to his plans to redevelop his own building, Holland testified.

When Holland refused, Leon and JACE pursued an everything-but-the-kitchen sink legal strategy, filing two successive zoning board cases while also dredging up an easement dating to the 1920s to prevent any change to the first two stories of the wall.

At one point, Holland also faced a criminal complaint, with Leon claiming that the developer had gained unauthorized access to the roof of his building at 1234 Washington Street. The claim was dismissed, with Holland making clear there was nothing to it, but by simply filing it in the first place, Leon endangered his bank financing for the project, Holland’s lawyers contend.

According to testimony by a cousin, Leon stated that he intended to drive Holland into bankruptcy and then buy his building at a bargain-basement price.

Leon certainly appears to have come close. Holland was forced to undertake an expensive redesign of the project, while dropping plans to convert the second floor, potentially the most profitable, from commercial use to condos.

All told, Holland contends that the dispute with Leon and JACE cost him nearly $12 million, according to court filings. That includes more than $5 million in increased costs, $6 million from the loss of plans to convert the building’s second floor to residential.

“The evidence is overwhelming that the Defendants commenced three civil suits and filed an application for a criminal complaint for ulterior motives and illegitimate purposes, designed to stop the development of 477 Harrison to gain an advantage in their desire to develop their property or to bankruptcy,” lawyers for Holland argued in a court filing.