The spotlight effect is the tendency to believe that more people notice outstanding characteristics of oneself than do. This is most true of an attribute that we are particularly concerned about. One reason this effect exists is egocentric bias. Humans tend to believe that their own thoughts are more valid than the thought of others. When your attention is focused on, for example, a stain on your shirt, you assume that his attention is focused correctly, and therefore the attention of most others will be focused on the stain as well. In reality, it's unlikely that others will notice that stain on your shirt, definitely not to the degree that you notice it. Most of the time, the attention of others will be focused on their own concerns and traits.
In a classic study by Gilovich and Medvec, subjects were instructed to wear a bright yellow t-shirt with a large picture of Barry Manilow’s head to a lecture. The subjects, on average, predicted that 50% of students in the lecture would notice their embarrassing t-shirt. Only 25% percent of their classmates noticed the t-shirt, far less than predicted.