It’s called ‘choice blindness’ apparently.  New research by Lars Hall looked into this phenomenon.

Imagine you sampled two jams, chose your favourite, and were then offered another taste of it before being asked to explain your preference. Would you notice that you’d been offered the wrong one, that you were actually tasting the jam you’d turned down?

The research found that just 20% of people realised that they’d been given a second helping of a different flavour than the one they picked as their favourite.

Abstract

We set up a tasting venue at a local supermarket and invited passerby shoppers to sample two different varieties of jam and tea, and to decide which alternative in each pair they preferred the most. Immediately after the participants had made their choice, we asked them to again sample the chosen alternative, and to verbally explain why they chose the way they did. At this point we secretly switched the contents of the sample containers, so that the outcome of the choice became the opposite of what the participants intended. In total, no more than a third of the manipulated trials were detected. Even for remarkably different tastes like Cinnamon-Apple and bitter Grapefruit, or the smell of Mango and Pernod was no more than half of all trials detected, thus demonstrating considerable levels of choice blindness for the taste and smell of two different consumer goods.

http://www.ncbi.nlm.nih.gov/pubmed/20637455