There were a couple of these liars who posed questions at the second Presidential debate the other night. The wording of the questions made it quite clear they had decided on a candidate and lied to be selected. The most egregious example of this type of "undecided" voter was the ponytail guy during the Bush the Elder/Clinton debate (think Carole Simpson was the moderator). He would have pulled a Lewinsky on Clinton on national television...certainly not "undecided."
As for pollsters, people who do not like to be confrontational will lie that they are "undecided" if they have selected a candidate who they perceive to be unpopular to the general public. For example, Klan member David Duke consistently underpolled compared to what he received at the ballot box.