The Seattle Times answers that very question:

After King County Elections officials uncovered what would later be described as the most egregious instance of voter-registration fraud in state history, John Jones was summoned to the office of then-King County Prosecutor Norm Maleng.
It was early last year and Maleng’s office was preparing to file charges against six canvassers who had filled out nearly 1,800 voter-registration cards the previous fall with names they made up using phone directories and books of baby names. Jones’ organization, the state chapter of the Association of Community Organizations for Reform Now – commonly known as ACORN – had paid the workers $8 an hour to sign up low-income voters in King and Pierce counties.
Maleng, King County’s Republican prosecutor who died in May 2007, pulled Jones aside for a man-to-man talk. Maleng told Jones that as a community leader, it wasn’t enough for him to admit fault and take responsibility for it: Jones must make sure the mistakes made in fall 2006 never, ever happen again.
Last week – as Republican presidential candidate John McCain continued to question the validity of the 1.3 million new voters ACORN has registered nationally for the November election — Jones said Maleng’s words are forever burned into his memory.
Maleng “said he respected the good work ACORN does but he made it clear that if there were any more problems [with voter registrations], we’d be having a very different conversation,” Jones said.
Since signing a settlement agreement with King County in summer 2007, the local ACORN chapter has remained under a microscope. For now, it has eliminated its mass voter-registration drives, registering only 500 voters this year compared with the 37,000 it registered for the last presidential election in 2004.
Still, Jones said – and state and county officials confirmed – that Washington ACORN hasn’t made a single mistake in adding its newest voters to county rolls.
“No problems have been associated with ACORN [this year], but you can bet people are watching. They’re on a short leash,” said David Ammons, a spokesman for Secretary of State Sam Reed.

37,000 in 2004. 500 in 2008.