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).
I have to wonder if many of the respondents in the survey didn't hold any position with much strength in the first place. Our society enforces the belief, not only that everyone is entitled to their opinions, but that everyone should have an opinion on just about any issue. People tend to stand by "opinions" that are really just snap judgments, which may be largely arbitrary.
If the respondents had little basis for determining their responses in the first place, it's unsurprising if they don't notice when they've been changed, and that it doesn't affect their ability to argue for them.
There is a long tradition in social science research, going back at least to Converse (1964), holding that most people's political views are relatively incoherent, poorly thought-through, and unstable. They're just making up responses to survey questions on the spot, in a way that can involve a lot of randomness.
This study demonstrates that plus confabulation, in a way that is particularly compelling because of the short time scale involved and the experimental manipulation of what opinion the person was defending.