Change blindness is the phenomenon whereby people fail to notice changes in scenery and whatnot if they're not directed to pay attention to it. There are countless videos online demonstrating this effect (one of my favorites here, by Richard Wiseman).
One of the most audacious and famous experiments is known informally as "the door study": an experimenter asks a passerby for directions, but is interrupted by a pair of construction workers carrying an unhinged door, concealing another person whom replaces the experimenter as the door passes. Incredibly, the person giving directions rarely notices they are now talking to a completely different person. This effect was reproduced by Derren Brown on British TV (here's an amateur re-enactment).
Subsequently a pair of Swedish researchers familiar with some sleight-of-hand magic conceived a new twist on this line of research, arguably even more audacious: have participants make a choice and quietly swap that choice with something else. People not only fail to notice the change, but confabulate reasons why they had preferred the counterfeit choice (video here). They called their new paradigm "Choice Blindness".
Just recently the same Swedish researchers published a new study that is even more shocking. Rather than demonstrating choice blindness by having participants choose between two photographs, they demonstrated the same effect with moral propositions. Participants completed a survey asking them to agree or disagree with statements such as "large scale governmental surveillance of e-mail and Internet traffic ought to be forbidden as a means to combat international crime and terrorism". When they reviewed their copy of the survey their responses had been covertly changed, but 69% failed to notice at least one of two changes, and when asked to explain their answers 53% argued in favor of what they falsely believed was their original choice, when they had previously indicated the opposite moral position (study here, video here).
Dark tactic: Have we verified that it doesn't work to present them with a paper saying what their opinion is even if they did NOT fill anything out? I explain how that might work This tactic is based on that possibility:
An unethical political candidate could have campaigners get a bunch of random people together and hand them a falsified survey with their name on it, making it look like they filled it out. The responses support a presidential candidate.
The unethical campaigner might then say: "A year ago, (too long for most people to remember the answers they gave on tests) you filled out a survey with our independent research company, saying you support X, Y and Z." If authoritative enough, they might believe this.
"These are the three key parts of my campaign! Can you explain why you support these?"
(victim explains)
"Great responses! Do you mind if we use these?"
(victim may feel compelled to say yes or seem ungrateful for the compliment)
"I think your family and friends should hear what great supports you have for your points on this important issue, don't you?"
(now new victims will be dragged in)
The responses that were given are used to make it look like there's a consensus.
For me at least, one year is also too long for me to reliably hold the same opinion, so if you did that to me, I think I'd likely say something like “Yeah, I did support X, Y and Z back then, but now I've changed my mind.” (I'm not one to cache opinions about most political issues -- I usually recompute them on the fly each time I need them.)