Long says the surprise increase threw his mortgage escrow account out of whack and comes at a time when he has struggled to make monthly house payments. He believes the city has effectively breached the contract it made when he bought the house and applied for the credit.

"I feel like I have a deal," he said. "I wasn't trying to skirt the system. I filled out a lot of paperwork to get what I got."

Long is hoping the city will restore his credit to its previous level, not just for this year but all four remaining years of the credit. "That would be the ultimately fair thing to do," he said.

City Councilman William H. Cole IV said the revelation of calculation errors points to a larger problem.

"My general frustration is that despite all the hard work the city and state have put into property taxes, there is still a lot of stuff to be fixed," he said. "We find inconsistencies all the time."

Cole said he sees a distinction between the two kinds of errors that Kraft said city officials described to him at a briefing last week.

In some cases, the state put into writing a pre-renovation assessment figure that the city now says should have been higher — meaning owners should have received a smaller tax break every year since the credit began.

Those homeowners "shouldn't be punished" with higher bills, Cole said. "I just think that's inherently unfair, particularly for people who based their affordability on that number."

But Cole agrees with the city in principle that homeowners should be taxed going forward if the state wrongly exempted not just the value of the renovation, but also market-driven appreciation. Even so, he said, he would need to see a "real world example" to make a firm conclusion.

City Council Vice President Edward Reisinger said state and city leaders should sit down together to discuss the problems and find solutions specific to the affected homeowners.

"Everybody's got to take a deep breath and look at this," he said. "If I was one of those people that the tax credit [was] the reason they bought the house, and now they owe thousands and thousands of dollars, that's not right. It sends a negative message out."

City finance director Harry E. Black said Baltimore officials were close to addressing all historic tax credits that have "issues."

"The state has the authority and responsibility for the assessing process," he said. "We're pretty confident we've been able to capture the universe of accounts that may have issues. We've not done 100 percent cleanup, but we're close."

Baltimore Sun reporter Yvonne Wenger contributed to this article.