Why were most of the General Election Polls Prediction wrong?

Most of the Polls forecast a hung Parliament but in truth there was always a probability of the Conservatives forming a Majority Government. Nevertheless, commentators focus on the Most Likely Event even though the uncertainty in the figures were wide. Hence Labour is ahead one day but the Conservatives are ahead by the same point the next. So it was a question of probability but there is another scientific explanation; The Observer Effect.

In science, the term observer effect refers to changes that the act of observation will make on a phenomenon being observed. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner. A commonplace example is checking the pressure in a car tire; this is difficult to do without letting out some of the air, thus changing the pressure.

So did the polls forecasting a hung parliament influence the outcome of the election? Some may argue yes. The polls indicating Labour could form a new government with SNP support did spice up the argument from David Cameron and the right of centre media to the voters. So did the polls not only forecast the election but unwittingly influence the outcome of the election as well?