It's not like there is only one thing to vote on on a given day. There are often dozens of votes covering a huge spectrum of issues often with very obtuse names and wording. So many people bring 'cheat sheets' so that they can remember what they're planning on voting on each issue. The problem in this case was that the order that issues where being voted on changed at the last minute and not everybody got the news and updated their notes. So it wasn't that they pressed the YES when they meant to press NO, its that they thought they where answering YES to question 36 when they where infact answering YES to question 37.