Non-existant people voting, higher voter turnout than people in the area, and people who didn't vote being recorded as having voted.

The most famous incident being this one where several hundred illegal immigrants voted-
http://www.cnn.com/ALLPOLITICS/1998/02/13/cq/sanchez.html

Now go on and belittle actual incidents of voter fraud while demanding more proof be offered despite your side saying any attempt to investigate voter fraud is racist.