The Des Moines Register offers an example of a dementia patient voting by absentee ballot:

Reports that an 87-year-old Council Bluffs woman was allegedly pressured to vote absentee by a Democratic campaign volunteer raise suspicions that some people who have diminished mental capacity are being targeted, a Republican official said Wednesday.

The allegation by Chris Taylor, a spokesman for the Republican National Committee, refers to claims by Angela Murphy-Hayes, who said her 87-year-old mother, a dementia patient, was coerced to cast a ballot about two weeks ago.

Must read election day blog, Election Journal originally broke the story with this video and testimony which involves Democratic field staffers coercing the vote out of Mrs. Garcia: