Sigmund Freud 
Psychology of Errors 
A General Introduction to Psychoanalysis (Easy to Read Layout)

Errors have a meaning and we may consider probable that every single mistake which occurs is meaningful.In these lectures I refer to the errors which an individual commits-as for example, errors of speech in which he wishes to say something and uses the wrong word; or those which happen to him in writing, and which he may or may not notice; or the case of misreading, in which one reads in the print or writing something different from what is actually there. A similar phenomenon occurs in those cases of mishearing what is said to one, where there is no question of an organic disturbance of the auditory function. Another series of such occurrences is based on forgetfulness-but on a forgetfulness which is not permanent, but temporary, as for instance when one cannot think of a name which one knows and always recognizes; or when one forgets to carry out a project at the proper time but which one remembers again later, and therefore has only forgotten for a certain interval.