Wisconsin Legislative Audit Bureau [PDF]:

We found that statutory requirements are not consistently followed. Among our survey respondents:

• only 85.3 percent of municipalities removed the names of inactive voters from their voter registration lists;
• only 71.4 percent sometimes or always notified registered voters before removing their names; and
• only 54.0 percent reported removing the names of ineligible felons.

Because of such inconsistencies, registration lists contain duplicate records and the names of ineligible individuals. For example, when
we reviewed more than 348,000 electronic voter registration records from eight municipalities, we identified 3,116 records that appear to show individuals who are registered more than once in the same municipality.

Greg Borowski and Stacy Forster have more:

Among the 348,000 electronic voter registration records checked were 105 potentially improper or fraudulent votes including:

• Ballots cast by 98 ineligible felons, including 57 in Madison.
• Two people who appear to have voted twice.
• Four cases of voters whose absentee ballots were included in official election results even though they died in the two weeks before the election.
• One instance of a 17-year-old in Madison who apparently voted.